Sample records for quantum dot coupled

We propose a method for implementation of a quantum computer using artificial molecules. The artificial molecule consists of two coupledquantumdots stacked along z direction and one single electron. One-qubit and two-qubit gates are constructed by one molecule and two coupled molecules, respectively.The ground state and the first excited state of the molecule are used to encode the |0> and |1> states of a qubit. The qubit is manipulated by a resonant electromagnetic wave that is applied directly to the qubit through a microstrip line. The coupling between two qubits in a quantum controlled NOT gate is switched on (off) by floating (grounding) the metal film electrodes. We study the operations of the gates by using a box-shaped quantumdot model and numerically solving a time-dependent Schridinger equation, and demonstrate that the quantum gates can perform the quantum computation. The operating speed of the gates is about one operation per 4ps. The reading operation of the output of the quantum computer can...

We propose a scheme to detect the chirality for a system consisting of three coupledquantumdots. The chirality is found to be determined by the frequency of the transition between chiral states under the chiral symmetry broken perturbation. The results are important to construct quantum gates and to demonstrate chiral entangle states in the triangle spin dots.

We have studied the polarization of surface and edge-emitted photoluminescence (PL) from structures with vertically coupled In0.5Ga0.5As/GaAs quantumdots (QD's) grown by molecular beam epitaxy. The PL polarization is found to be strongly dependent on the number of stacked layers. While single...... number due to increasing dot size....

Transport through symmetric parallel coupledquantumdot system has been studied, using non-equilibrium Green function formalism. The inter-dot tunnelling with on-dot and inter-dot Coulomb repulsion is included. The transmission coefficient and Landaur–Buttiker like current formula are shown in terms of internal states of quantumdots. The effect of inter-dot tunnelling on transport properties has been explored. Results, in intermediate inter-dotcoupling regime show signatures of merger of two dots to form a single composite dot and in strong coupling regime the behaviour of the system resembles the two decoupled dots.

We prepare a gate-defined quadruple quantumdot to study the gate-tunability of single to quadruple quantumdots with finite inter-dot tunnel couplings. The measured charging energies of various double dots suggest that the dot size is governed by the gate geometry. For the triple and quadruple dots, we study the gate-tunable inter-dot tunnel couplings. For the triple dot, we find that the effective tunnel coupling between side dots significantly depends on the alignment of the center dot potential. These results imply that the present quadruple dot has a gate performance relevant for implementing spin-based four-qubits with controllable exchange couplings.

. An alternative approach is to couple the quantumdot directly to the propagating mode of a photonic waveguide. We demonstrate the coupling of single quantumdots to a photonic crystal waveguide using time-resolved spontaneous emission measurements. A pronounced effect is seen in the decay rates of dotscoupled...

. An alternative approach is to couple the quantumdot directly to the propagating mode of a photonic waveguide. We demonstrate the coupling of single quantumdots to a photonic crystal waveguide using time-resolved spontaneous emission measurements. A pronounced effect is seen in the decay rates of dotscoupled...

In this study, interaction between core-shells nanoparticles and quantumdots is discussed via the full-quantum-theory method. The electromagnetic field of the nanoparticles is derived by the quasistatic approximation method and the results for different regions of the nanoparticles are quantized from the time-harmonic to the wave equation. Utilizing the optical field quantization, the nanoparticles' and quantumdots' deriving amplitudes contributing to the excitation waves are determined. In the current model, two counterpropagating waves with two different frequencies are applied. We derived the Maxwell-Bloch equations from the Heisenberg-Langevin equations; thus the nanoparticles-quantumdots interaction is perused. Moreover, by full quantum analyzing of the analytical expression, the quantum-plasmonic coupling relation and the Purcell factor are achieved. We show that the spontaneous emission of quantumdots can be dramatically manipulated by engineering the plasmon-plasmon interaction in the core-shells nanoparticles. This issue is a very attractive point for designing a wide variety of quantum-plasmonic sensors. Through the investigation of the nanoparticle plasmonic interaction effects on absorbed power, the results show that the nanoparticles' and quantumdots' absorption saturation state can be switched to each other just by manipulation of their deriving amplitudes. In fact, we manage the interference between the two waves' deriving amplitudes just by the plasmonic interactions effect.

A solid state model for quantum computation has potential advantages in terms of the ease of fabrication, characterization, and integration. The fundamental requirements for a quantum computer involve the realization of basic processing units (qubits), and a scheme for controlled switching and coupling among the qubits, which enables one to perform controlled operations on qubits. We propose a model for quantum computation based on optically coupledquantumdot arrays, which is computationally similar to the atomic model proposed by Cirac and Zoller. In this model, individual qubits are comprised of two coupledquantumdots, and an array of these basic units is placed in an optical cavity. Switching among the states of the individual units is done by controlled laser pulses via near field interaction using the NSOM technology. Controlled rotations involving two or more qubits are performed via common cavity mode photon. We have calculated critical times, including the spontaneous emission and switching times, and show that they are comparable to the best times projected for other proposed models of quantum computation. We have also shown the feasibility of accessing individual quantumdots using the NSOM technology by calculating the photon density at the tip, and estimating the power necessary to perform the basic controlled operations. We are currently in the process of estimating the decoherence times for this system; however, we have formulated initial arguments which seem to indicate that the decoherence times will be comparable, if not longer, than many other proposed models.

Here we report the fabrication and quantum transport measurements of gates controlled parallel-coupled bilayer graphene double quantumdot. It is shown that the interdot coupling strength of the parallel double dots can be effectively tuned from weak to strong regime by both the in-plane plunger gates and back gate. All the relevant energy scales and parameters of the bilayer graphene parallel-coupled double dot can be extracted from the honeycomb charge stability diagrams revealed through the transport measurements.

A quantum bit computing architecture includes a plurality of single spin memory donor atoms embedded in a semiconductor layer, a plurality of quantumdots arranged with the semiconductor layer and aligned with the donor atoms, wherein a first voltage applied across at least one pair of the aligned quantumdot and donor atom controls a donor-quantumdotcoupling. A method of performing quantum computing in a scalable architecture quantum computing apparatus includes arranging a pattern of single spin memory donor atoms in a semiconductor layer, forming a plurality of quantumdots arranged with the semiconductor layer and aligned with the donor atoms, applying a first voltage across at least one aligned pair of a quantumdot and donor atom to control a donor-quantumdotcoupling, and applying a second voltage between one or more quantumdots to control a Heisenberg exchange J coupling between quantumdots and to cause transport of a single spin polarized electron between quantumdots.

are tuned electrostatically. This includes tuning the odd occupation of the dot through a quantum phase transition, where it forms a singlet with excitations in the superconductor. We detail the fabrication of these bottom gated devices, which additionally feature ancillary sensor dots connected...

Controlling long-distance quantum correlations is central to quantum computation and simulation. In quantumdot arrays, experiments so far rely on nearest-neighbour couplings only, and inducing long-distance correlations requires sequential local operations. Here, we show that two distant sites can be tunnel-coupled directly. The coupling is mediated by virtual occupation of an intermediate site, with a strength that is controlled via the energy detuning of this site. It permits a single charge to oscillate coherently between the outer sites of a triple dot array without passing through the middle, as demonstrated through the observation of Landau-Zener-Stückelberg interference. The long-distance coupling significantly improves the prospects of fault-tolerant quantum computation using quantumdot arrays, and opens up new avenues for performing quantum simulations in nanoscale devices.

The competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction is investigated in a double quantumdots system, coupled via a central open conducting region. A perpendicular magnetic field induces the formation of Landau Levels which in turn give rise to the so-called Kondo chessboard pattern in the transport through the quantumdots. The two quantumdots become therefore chirally coupled via the edge channels formed in the open conducting area. In regions where both quantumdots exhibit Kondo transport the presence of the RKKY exchange interaction is probed by an analysis of the temperature dependence. The thus obtained Kondo temperature of one dot shows an abrupt increase at the onset of Kondo transport in the other, independent of the magnetic field polarity, i.e. edge state chirality in the central region.

The effect of phase-breaking process on the ac response of a coupled double quantumdot is studied in this paper based on the nonequilibrium Green function formalism. A general expression is derived for the ac current in the presence of electron-phonon interaction. The ac conductance is numerically computed and the results are compared with those in [Anatram M P and Datts S 1995 Phys. Rev. B 51 7632]. Our results reveal that the inter-dot electron tunnelling interplays with that between dots and electron reservoirs, and contributes prominently to the ac current when inter-dot tunnelling coupling is much larger than the tunnelling coupling between dots and electron reservoirs. In addition, the phase-breaking process is found to have a significant effect on the ac transport through the coupled double dot.

In this thesis, sub-gap states in bottom-gated InAs N–dot–S, N–double dot–S, and N–dot–S–dot–N devices are investigated, and several different theories are developed to model these states. Experimental results include tracking single levels of the dot in an N–dot–S device as the tunnel couplings ...... process, which is based on the general properties of Krylov subspaces alone, and is thus not tied to a specific physical system....... with floating gates. A numerical technique is developed, which predicts the position of Yu-Shiba-Rusinov sub-gap states in the proximitized Anderson model as well as properties of these states. This theory is valid for all occupations of the dot and for weak to intermediate coupling. We compare...

We study theoretically Coulomb drag in capacitively coupledquantumdots (CQDs) -- a biasdriven dotcoupled to an unbiased dot where transport is due to Coulomb mediated energy transfer drag. To this end, we introduce a master-equation approach which accounts for higher-order tunneling (cotunneling....... Interestingly, the direction of the drag current is not determined by the drive current, but by an interplay between the energy-dependent lead couplings. Studying the drag mechanism in a graphene-based CQD heterostructure, we show that the predictions of our theory are consistent with recent experiments...

In this paper, we develop a theory for discrete solitons interaction with a string of coupledquantumdots in view of the local field effects. Discrete nonlinear Schrodinger (DNLS) equations are used to describe the dynamics of the string. Numerical calculations are carried out and results are analyzed with the help of matlab software. With the help of numerical solutions we demonstrate that in the quantumdots string, Rabi oscillations (RO) are self trapped into stable bright Rabi solitons. The Rabi oscillations in different types of nanostructures have potential applications to the elements of quantum logic and quantum memory.

Herein, we report the first demonstration of room temperature enhanced light-matter coupling in the visible regime for metamaterials using cooperative coupled quasi two dimensional quantumdot assemblies located at precise distances from the hyperbolic metamaterial (HMM) templates. The non-monotonic variation of the magnitude of strong coupling, manifested in terms of strong splitting of the photoluminescence of quantumdots, can be explained in terms of enhanced LDOS near the surface of such metamaterials as well as the plasmon mediated super-radiance of closely spaced quantumdots (QDs). Our methodology of enhancing broadband, room temperature, light-matter coupling in the visible regime for metamaterials opens up new possibilities of utilising these materials for a wide range of applications including QD based thresholdless nanolasers and novel metamaterial based integrated photonic devices.

Quantumdots are studied as artificial atoms for building novel artificial solids, as nanodevices in nanoelectronics, and as bio/nanohybrids. We present an atomistic tight-binding theory of coupled CdS nanocrystals and vertically and laterally coupled InAs self-assembled dots. Electron states of coupleddots follow the analogy of coupleddots as artificial molecules. Symmetric/antisymmetric pairs are formed with strongest coupling between states with high density at interdot interfaces. Complex coupling of hole states, with significant departures from the artificial molecule analogy, occurs because the coupling is determined by the hole envelope function and the hole atomic state. Some hole states couple to form symmetric/antisymmetric pairs. Other hole states couple through additional intermediate states to form two strongly split symmetric states and an antisymmetric state insensitive to coupling. These coupling effects lead to level reordering, changes in state symmetry, conversion of dark states to bright states and vice versa, and tailored polarization dependence.

We investigate coherent single surface-plasmon transport in a metal nanowire strongly coupled to two colloidal quantumdots. Analytical expressions are obtained for the transmission and reflection coefficients by solving the corresponding eigenvalue equation. Remote entanglement of the wave functions of the two quantumdots can be created if the inter-dot distance is equal to a multiple half-wavelength of the surface plasmon. Furthermore, by applying classical laser pulses to the quantumdots...

We demonstrate selective optical coupling of multiple, site controlled semiconductor quantumdots (QDs) to photonic crystal waveguide structures. The impact of the exact position and emission spectrum of the QDs on the coupling efficiency is elucidated. The influence of optical disorder and end...

We study the coupling between a photonic crystal cavity and an off-resonant quantumdot under resonant excitation of the cavity or the quantumdot. Linewidths of the quantumdot and the cavity as a function of the excitation laser power are measured. We show that the linewidth of the quantumdot, measured by observing the cavity emission, is significantly broadened compared to the theoretical estimate. This indicates additional incoherent coupling between the quantumdot and the cavity.

We carried out a nested Schrieffer-Wolff transformation of an Anderson two-impurity Hamiltonian to study the spin-spin coupling between two dynamical quantumdots under the influence of rotating transverse magnetic field. As a result of the rotating field, we predict a novel Ising type spin-spin coupling mechanism between quantumdots, whose strength is tunable via the magnitude of the rotating field. Due to its dynamical origin, this new coupling mechanism is qualitatively different from the all existing static couplings such as RKKY, while the strength could be comparable to the strength of the RKKY coupling. The dynamical coupling with the intristic RKKY coupling enables to construct a four level system of maximally entangled Bell states in a controllable manner. This work was supported by the special funds for the Major State Basic Research Project of China (No. 2011CB925601) and the National Natural Science Foundation of China (Grants No. 11004036 and No. 91121002).

The energy spectra of low-lying states of an exciton in a single and a vertically coupledquantumdots are studied under the influence of a perpendicularly applied magnetic field. Calculations are made by using the method of numerical diagonalization of the Hamiltonian within the effective-mass approximation. We also calculated the binding energy of the ground and the excited states of an exciton in a single quantumdot and that in a vertically coupledquantumdot as a function of the dot radius for different vaJues of the distance and the magnetic field strength.

InAs quantumdots have been placed at different distances to a silver mirror. We extract the coupling of quantumdots to surface plasmon polaritons as a function of the distance by time-resolved spontaneous emission measurements.......InAs quantumdots have been placed at different distances to a silver mirror. We extract the coupling of quantumdots to surface plasmon polaritons as a function of the distance by time-resolved spontaneous emission measurements....

A system of two laterally coupledquantumdots with modified Gaussian potential has been considered. Each quantumdot has an electron under electric and magnetic field. The quantumdots have been considered as hydrogen-like atoms. The physical picture has translated into the Heisenberg spin Hamiltonian. The Schrödinger equation using finite element method has been numerically solved. The exchange energy factor has been calculated as a functions of electric field, magnetic field, and the separation distance between the centers of the dots ( d). According to the results, it is found that there is the transition from anti-ferromagnetic to ferromagnetic for constant electric field. Also, the transition occurs from ferromagnetic to anti-ferromagnetic for constant magnetic field (B>1 T). With decreasing the distance between the centers of the dots and increasing magnetic field, the transition occurs from anti-ferromagnetic to ferromagnetic. It is found that a switching of exchange energy factor is presented without canceling the interactions of the electric and magnetic fields on the system.

The theoretical investigation of the spectrum of electrons, holes, and excitons in the superlattice of cylindrical quantumdots with weakest coupling of quasiparticles between vertical layers of quantumdots is carried out. The calculations are fulfilled by the example of cylindrical quantumdots of beta-HgS introduced into beta-CdS as the superlattice. It is shown that electron and hole in such system form quasi-two-dimensional energy minibands, but excitons are described by the Sugano-Shinada model. The dependence of quasiparticle spectra on geometric parameters of the superlattice with cylindrical quantumdots is studied. It is shown that the position of minibands of all quasiparticles is very sensitive to variation of the quantumdot height

It is well known that a parallel quantum computer is more powerful than a classical one. So far, there are some important works about the construction of universal quantum logic gates, the key elements in quantum computation. However, they are focused on operating on one degree of freedom (DOF) of quantum systems. Here, we investigate the possibility of achieving scalable hyper-parallel quantum computation based on two DOFs of photon systems. We construct a deterministic hyper-controlled-not (hyper-CNOT) gate operating on both the spatial-mode and the polarization DOFs of a two-photon system simultaneously, by exploiting the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics (QED). This hyper-CNOT gate is implemented by manipulating the four qubits in the two DOFs of a two-photon system without auxiliary spatial modes or polarization modes. It reduces the operation time and the resources consumed in quantum information processing, and it is more robust against the photonic dissipation noise, compared with the integration of several cascaded CNOT gates in one DOF. PMID:24721781

In Coulomb drag, a current flowing in one conductor can induce a voltage across an adjacent conductor via the Coulomb interaction. The mechanisms yielding drag effects are not always understood, even though drag effects are sufficiently general to be seen in many low-dimensional systems. In this Letter, we observe Coulomb drag in a Coulomb-coupled double quantumdot and, through both experimental and theoretical arguments, identify cotunneling as essential to obtaining a correct qualitative understanding of the drag behavior.

The energy spectra of low-lying states of an exciton in a single and a vertically coupledquantumdots arestudied under the influence of a perpendicularly applied magnetic field. Calculations are made by using the method ofnumerical diagonalization of the Hamiltonian within the effective-mass approximation. We also calculated the bindingenergy of the ground and the excited states of an exciton in a single quantumdot and that in a vertically coupledquantum dot as a function of the dot radius for different values of the distance and the magnetic field strength.

We present time-resolved spontaneous emission measurements of single quantumdots embedded in photonic crystal waveguides. Quantumdots that couple to a photonic crystal waveguide are found to decay up to 27 times faster than uncoupled quantumdots. From these measurements -factors of up to 0.89 ...

We study a two-electron system in a double-layer quantumdot under a magnetic field by means of the exact diagonalization of the Hamiltonian matrix.We find that discontinuous ground-state energy transitions are induced by an external magnetic field in the case of strong coupling.However,in the case of weak coupling,the angular momentum L of the true ground state does not change in accordance with the change of the magnetic field B and remains L=0.

This book reviews recent advances in the exciting and rapidly growing field of semiconductor quantumdots via contributions from some of the most prominent researchers in the scientific community. Special focus is given to optical, quantum optical, and spin properties of single quantumdots due to their potential applications in devices operating with single electron spins and/or single photons. This includes single and coupledquantumdots in external fields, cavity-quantum electrodynamics, and single and entangled photon pair generation. Single Semiconductor QuantumDots also addresses growth techniques to allow for a positioned nucleation of dots as well as applications of quantumdots in quantum information technologies. (orig.)

We consider the subgap physics of a hybrid double-quantumdot Cooper-pair splitter with large single-level spacings, in the presence of tunneling between the dots and finite Coulomb intra- and interdot Coulomb repulsion. In the limit of a large superconducting gap, we treat the coupling of the dots to the superconductor exactly. We employ a generalized master-equation method, which easily yields currents, noise, and cross-correlators. In particular, for finite inter- and intradot Coulomb interaction, we investigate how the transport properties are determined by the interplay between local and nonlocal tunneling processes between the superconductor and the dots. We examine the effect of interdot tunneling on the particle-hole symmetry of the currents with and without spin-orbit interaction. We show that spin-orbit interaction in combination with finite Coulomb energy opens the possibility to control the nonlocal entanglement and its symmetry (singlet/triplet). We demonstrate that the generation of nonlocal entanglement can be achieved even without any direct nonlocal coupling to the superconducting lead.

Coupling a single quantum emitter, such as a fluorescent molecule or a quantumdot (QD), to a plasmonic nanostructure is an important issue in nano-optics and nano-spectroscopy, relevant for a wide range of applications, including tip-enhanced near-field optical microscopy, plasmon enhanced molecular sensing and spectroscopy, and nanophotonic amplifiers or nanolasers, to mention only a few. While the field enhancement of a sharp nanoantenna increasing the excitation rate of a very closely positioned single molecule or QD has been well investigated, the detailed physical mechanisms involved in the emission of a photon from such a system are, by far, less investigated. In one of our ongoing research projects, we try to address these issues by constructing and spectroscopically analysing geometrically simple hybrid heterostructures consisting of sharp gold cones with single quantumdots attached to the very tip apex. An important goal of this work is to tune the longitudinal plasmon resonance by adjusting the cones' geometry to the emission maximum of the core-shell CdSe/ZnS QDs at nominally 650 nm. Luminescence spectra of the bare cones, pure QDs and hybrid systems were distinguished successfully. In the next steps we will further investigate, experimentally and theoretically, the optical properties of the coupled systems in more detail, such as the fluorescence spectra, blinking statistics, and the current results on the fluorescence lifetimes, and compare them with uncoupled QDs to obtain a clearer picture of the radiative and non-radiative processes.

The fabrication, by pure self-assembly, of GaAs/AlGaAs dot-ring quantum nanostructures is presented. The growth is performed via droplet epitaxy, which allows for the fine control, through As flux and substrate temperature, of the crystallization kinetics of nanometer scale metallic Ga reservoirs deposited on the surface. Such a procedure permits the combination of quantumdots and quantum rings into a single, multi-functional, complex quantum nanostructure.

refrigeration with coupledquantum wells. Many photonic quantum information processing applications would benet from a highbrightness, ber-coupled source of triggered single photons. This thesis presents a study of such sources based on quantumdotscoupled to unidirectional photonic-crystal waveguide devices......This thesis presents the study of solid-state quantum emitters in two dierent forms. The rst part of the thesis deals with quantumdot based single-photon sources with an emphasis on ecient photon extraction into an optical ber. The second part of the thesis covers a theoretical study of optical...... the characterization of single quantumdots. The second method, evanescent coupling from a tapered waveguide to a microber, demonstrates a chip-to-ber coupling eciency exceeding 80 % in passive re- ection measurements. The characterization of quantumdots from this device establishes a ber-coupled source eciency of 15...

We investigate antilocalization due to spin-orbit coupling in ballistic GaAs quantumdots. Antilocalization that is prominent in large dots is suppressed in small dots, as anticipated theoretically. Parallel magnetic fields suppress both antilocalization and also, at larger fields, weak...... localization, consistent with random matrix theory results once orbital coupling of the parallel field is included. In situ control of spin-orbit coupling in dots is demonstrated as a gate-controlled crossover from weak localization to antilocalization....

We study a four-electron system in a vertically coupled four-layer quantumdot under a magnetic field by the exact diagonalization of the Hamiltonian matr/x. We find that discontinuous ground-state energy transitions are induced by an external magnetic field. We find that dot-dot distance and electron-electron interaction strongly affect the low-lying states of the coupledquantumdots. The inter-dot correlation leads to some sequences of possible disappearances of ground state transitions, which are present for uncoupled dots.

We study a four-electron system in a vertically coupled four-layer quantumdot under a magnetic field by the exact diagonalization of the Hamiltonian matrix. We find that discontinuous ground-state energy transitions are induced by an external magnetic field. We find that dot-dot distance and electron-electron interaction strongly affect the low-lying states of the coupledquantumdots. The inter-dot correlation leads to some sequences of possible disappearances of ground state transitions, which are present for uncoupled dots.

We investigate antilocalization due to spin-orbit coupling in ballistic GaAs quantumdots. Antilocalization that is prominent in large dots is suppressed in small dots, as anticipated theoretically. Parallel magnetic fields suppress both antilocalization and also, at larger fields, weak localizat......We investigate antilocalization due to spin-orbit coupling in ballistic GaAs quantumdots. Antilocalization that is prominent in large dots is suppressed in small dots, as anticipated theoretically. Parallel magnetic fields suppress both antilocalization and also, at larger fields, weak...

Ultrafast density-dependent optical spectroscopic measurements on a quantumdots-in-a-well heterostructure reveal several distinctive phenomena, most notably a strong coupling between the quantum well population and light absorption at the quantumdot excited state.

We have studied the far-infrared spectra of two-electron vertically coupledquantumdots in an axial magnetic field by exact diagonalization. The calculated results show an obvious difference in role between the interactions for spin S = 1 and for spin S = 0. The results support the possibility to evaluate the interactions by far-infrared spectroscopy in vertically coupledquantumdots.

In this paper, we report a full-wavelength plasmonic dipole optical antenna coupledquantumdot infrared photodetector (QDIP). The plasmonic dipole optical antenna can effectively modify the EM wave distribution and convert free-space propagation infrared light to localized surface plasmonic resonance (SPR) within the nanometer (nm) gap region of the full-wavelength dipole antenna. The plasmonic dipole optical antenna coupled QDIP shows incident-angle-dependent photocurrent enhancement. The angular dependence follows the far-field pattern of a full-wavelength dipole antenna. The directivity of the plasmonic dipole optical antenna is measured to be 1.8 dB, which agrees well with the antenna simulation. To our best knowledge, this is the first report of the antenna far-field and directivity measurement. The agreement of the detection pattern and the directivity with antenna theory confirms functions of an optical antenna are similar to that of a RF antenna.

The gate-defined quantumdot is regarded as one of the basic structures required for scalable semiconductor quantum processors. Here, we demonstrate a structure that contains three quantumdots scaled in series. The electron number of each dot and the tunnel coupling between them can be tuned conveniently using splitting gates. We tune the quantumdot array asymmetrically such that the tunnel coupling between the right dot and the central dot is much larger than that between the left dot and the central dot. When driven by microwaves, the sidebands of the photon-assisted tunneling process appear not only in the left-to-central dot transition region but also in the left-to-right dot transition region. These sidebands are both attributed to the left-to-central transition for asymmetric coupling. Our result shows that there is a region of a triple quantumdot structure that remains indistinct when studied with a normal two-dimensional charge stability diagram; this will be helpful in future studies of the scalability of quantumdot systems.

The weak classical light excitations in many semiconductor quantumdots have been chosen as important solidstate quantum systems for processing quantum information and implementing quantum computing. For strong classical light we predict theoretically a novel phase transition as a function of magnitude of this classical light from the deformed to the normal phases in resonance case, and the essential features of criticality such as the scaling behaviour, critical exponent and universality are also present in this paper.

The exact entanglement dynamics in a hybrid structure consisting of two quantumdots (QDs) in the proximity of a metal nanoshell is investigated. Nanoshells can enhance the local density of states, leading to a strong-coupling regime where the excitation energy can coherently be transferred between the QDs and the nanoshell in the form of Rabi oscillations. The long-lived entangled states can be created deterministically by optimizing the shell thickness as well as the ratio of the distances between the QDs and the surface of the shell. The loss of the system is greatly reduced even when the QDs are ultraclose to the shell, which signifies a slow decay rate of the coherence information and longtime entanglement preservation. Our protocol allows for an on-demand, fast, and almost perfect entanglement even at strong carrier-phonon interaction where other systems fail.

Rectification of thermal fluctuations in mesoscopic conductors is the key idea behind recent attempts to build nanoscale thermoelectric energy harvesters to convert heat into useful electric power. So far, most concepts have made use of the Seebeck effect in a two-terminal geometry, where heat and charge are both carried by the same particles. Here, we experimentally demonstrate the working principle of a new kind of energy harvester, proposed recently, using two capacitively coupledquantumdots. We show that, due to the novel three-terminal design of our device, which spatially separates the heat reservoir from the conductor circuit, the directions of charge and heat flow become decoupled. This enables us to manipulate the direction of the generated charge current by means of external gate voltages while leaving the direction of heat flow unaffected. Our results pave the way for a new generation of multi-terminal nanoscale heat engines.

Full Text Available We theoretically investigate the electron transport properties of a wheel-like quantumdot (QD structure with a central QD side coupled with many pairs of QD and Majorana bound states (MBSs by using the nonequilibrium Green’s function method. For clarity, we concentrate our researches on the parameter regime where interdot couplings is much smaller than the inter-MBS and MBS-QD couplings, which ensures the conductance peaks induced by them distinguishable. In the absence of the interdot couplings among the side QDs, the increase of the MBS-QD pair number is equivalent to the increase of the interdot coupling in the QD structure including one central QD and one MBS-QD pair. It is shown that as a response the interval between two side symmetrical peaks will be enlarged, and the MBS-QD couplings will bring into being a zero-bias conductance peak which can be split into two symmetrical sub-peaks by the nonzero inter-MBS couplings. In the presence of the interdot couplings among the side QDs, they make serious influences on the conductance peaks determined by the QD energy levels, and still comes into being the zero-bias conductance peak due to the MBS-QD couplings, yet which is split into two asymmetrical sub-peaks under the influences of the nonzero inter-MBS couplings. Moreover, we conduct a detailed investigation into how the couplings among side QDs affect the transport properties, clearly exposing the underneath mechanics responsible for producing these phenomena. Finally, a generalization is made so as to discuss the geometry universality and the parameter universality of the conclusion drawn in the present work. It should be emphasized that this research will be helpful for a comprehensive understanding the quantum transport through the QD systems coupled with MBSs.

Self-assembled quantumdot (QD) grown by molecular beam epitaxy implements the ultimate quantum confined structure in the solid-state. Because electrons are confined in all three dimensions, QDs have shown delta-function-like energy dependence of the density of states. The identification of QDs as artificial atoms has been proven by the observation of fundamental exciton emission with perfect photon antibunching and with a linewidth limited by spontaneous emission. As artificial atoms that can be monolithically integrated with other functional devices in the setting of semiconductor technology, QDs have been the source of inspiration of several novel quantum devices. Nonetheless, QD self-assemble on random nucleation sites, whereas many proposals require precise positioning of a single QD. In this thesis, two new elements of design and control for coupling single QDs to single nanostructure will be presented: (i) a technique to control ab-initio the site nucleation of a single QD and (ii) a deterministic approach to the coupling of single QDs to single nanocavity modes. Chapter 1 is dedicated to a theoretical overview of the QD system, first, as a solid-state heterostructure that confines electrons and, second, as an artificial atom interacting with confined photons. Chapter 2 presents a technique to grow highly ordered QD arrays by engineering subsurface periodic strain templates. This technique achieved almost perfect registering of single QDs into a two-dimensional lattice. In Chapter 3 a deterministic approach to the realization of solid-state cavity quantum electrodynamics (QED) systems is demonstrated. Based on a precise spatial and spectral matching between a single QD and a PC microcavity, this approach yielded cavity QED effects (such as Purcell effect) in all fabricated nanostructures.

We present Monte Carlo wavefunction simulations for quantum computations employing an exchange-coupled array of quantumdots. Employing a combination of experimentally and theoretically available parameters, we find that gate fidelities greater than 98 % may be obtained with current experimental and technological capabilities. Application to an encoded 3 qubit (nine physical qubits) Deutsch-Josza computation indicates that the algorithmic fidelity is more a question of the total time to implement the gates than of the physical complexity of those gates.

The stationary tunneling current and differential conductance of the coupledquantumdots system with split-gates are calculated by generalizing the Beenaker’s linear response theory for the description of the Coulomb-blockade oscillations of the conductance in the single quantumdot.The calculation of the charging diagram in parallel through the double dot as function of the two side-gate voltages shows a remarkable agreement with the recent experimental results by Hatano et al.

The influence of the electron-LO-phonon coupling on energy spectrum of the low-lying states ofan exciton inparabolic quantumdots is investigated as a function of dot size. Calculations are made by using the method of few-bodyphysics within the effective-mass approximation. A considerable decrease of the energy in the stronger confinement rangeis found for the low-lying states of an exciton in quantumdots, which results from the confinement of electron-phononcoupling.

Based on the character of semiconductors and the structure of optical fiber coupler,a new amplifying fiber,coupled semiconductor quantumdot amplifying fiber (CSQDAF),has been presented. A simplified model of PbS quantumdot amplifying fiber is built on the energy band structure of semiconductor quantumdots,and a simple expression deduced from the two-level rate equations and light propagation equations is shown in this paper,by which the gain of quantumdot amplifying fiber can be calculated. A gain of approximately 4.5 dB has been measured in this coupled semiconductor quantumdot amplifying fiber at a wavelength of 1310 nm,when the fiber is pumped by a laser operating at a wavelength of 980 nm with power of 30 mW.

We propose an experimental scheme to implement a strong photon blockade with a single quantumdotcoupled to a nanocavity. The photon blockade effect can be tremendously enhanced by driving the cavity and the quantumdot simultaneously with two classical laser fields. This enhancement of photon blockade is ascribed to the quantum interference effect to avoid two-photon excitation of the cavity field. Comparing with Jaynes-Cummings model, the second-order correlation function at zero time delay $g^{(2)}(0)$ in our scheme can be reduced by two orders of magnitude and the system sustains a large intracavity photon number. A red (blue) cavity-light detuning asymmetry for photon quantum statistics with bunching or antibunching characteristics is also observed. The photon blockade effect has a controllable flexibility by tuning the relative phase between the two pumping laser fields and the Rabi coupling strength between the quantumdot and the pumping field. Moreover, the photon blockade scheme based on quantum in...

The Fano resonance of a quantum wire (QW) with a side-coupledquantumdot (QD) is investigated. The QD has multilevel and is in the Coulomb blockade regime. We show that there are two aspects in contribution to asymmetric Fano dip line shape of conductance: (1) the quantum interference between the resonant level and non-resonant levels, (2) the asymmetric electron occupation of levels in the two sides of a resonant level in the QD. The smearing of the asymmetry of the dip structure with the increasing temperature is partially attributed to fluctuation of electron state in the QD.

We demonstrate that the spontaneous emission decay rate of semiconductor quantumdots can be strongly modified by the coupling to disorder-induced Anderson-localized photonic modes. We experimentally measure, by means of time-resolved photoluminescence spectroscopy, the enhancement...... of the spontaneous emission decay rate by up to a factor 15 and an efficiency of channeling single photons into Anderson-localized modes reaching values as high as 94%. These results prove that disordered photonic media provide an efficient platform for quantum electrodynamics, offering a novel route to quantum...

We investigate the effects of spin-orbit interaction (SOI) on the exchange cotunneling through a spinful Coulomb blockaded quantumdot. In the case of zero magnetic field, Kondo effect is shown to take place via a Kramers doublet and the SOI will merely affect the Kondo temperature. In contrast, we...

We investigate the effects of spin-orbit interaction (SOI) on the exchange cotunneling through a spinful Coulomb blockaded quantumdot. In the case of zero magnetic field, Kondo effect is shown to take place via a Kramers doublet and the SOI will merely affect the Kondo temperature. In contrast, ...

The interaction of a single quantumdot with a bowtie antenna is demonstrated for visible light. The antenna is generated at the apex of a Si3N4 atomic force microscopy tip by focused ion beam milling. When scanned over the quantumdot, its photoluminescence is enhanced while its excited-state lifetime is decreased. Our observations demonstrate that the relaxation channels of a single quantum emitter can be controlled by coupling to an efficiently radiating metallic nanoantenna.

In this paper, we analyze the performance of an electro-optic modulator based on a single quantumdot strongly coupled to a nano-resonator, where electrical control of the quantumdot frequency is achieved via quantum confined Stark effect. Using realistic system parameters, we show that modulation speeds of a few tens of GHz are achievable with this system, while the energy per switching operation can be as small as 0.5 fJ. In addition, we study the non-linear distortion, and the effect of pure quantumdot dephasing on the performance of the modulator.

The electronic structures of coupledquantumdots grown on (11N)-oriented substrates are studied in the framework of effective-mass envelope-function theory. The results show that the all-hole subbands have the smallest widths and the optical properties are best for the (113), (114), and (115) growth directions. Our theoretical results agree with the available experimental data. Our calculated results are useful for the application of coupledquantumdots in photoelectric devices.

Plasmonic antennas are key elements to control the luminescence of quantum emitters. However, the antenna's influence is often hidden by quenching losses. Here, the luminescence of a quantumdotcoupled to a gold dimer antenna is investigated. Detailed analysis of the multiply excited states quantifies the antenna's influence on the excitation intensity and the luminescence quantum yield separately.

Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantumdot and a trapped ion and we demonstrate that single photons generated by a quantumdot controllably change the internal state of an $\\textrm{Yb}^+$ ion. We ameliorate the effect of the sixty-fold mismatch of the radiative linewidths with coherent photon generation and a high-finesse fiber-based optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the $\\sigma_z$-projection of the quantum-dot spin and the internal state of the ion provides a promising step towards quantum state-transfer in a hybrid photonic network.

Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantumdot and a trapped ion and we demonstrate that single photons generated by a quantumdot controllably change the internal state of a Yb^{+} ion. We ameliorate the effect of the 60-fold mismatch of the radiative linewidths with coherent photon generation and a high-finesse fiber-based optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the σ_{z} projection of the quantum-dot spin and the internal state of the ion provides a promising step towards quantum-state transfer in a hybrid photonic network.

We present an approach for entangling electron spin qubits localized on spatially separated impurity atoms or quantumdots via a multielectron, two-level quantumdot. The effective exchange interaction mediated by the dot can be understood as the simplest manifestation of Ruderman-Kittel-Kasuya-Yosida exchange, and can be manipulated through gate voltage control of level splittings and tunneling amplitudes within the system. This provides both a high degree of tunability and a means for realizing high-fidelity two-qubit gates between spatially separated spins, yielding an experimentally accessible method of coupling donor electron spins in silicon via a hybrid impurity-dot system.

We study the optical emission of single semiconductor quantumdots weakly coupled to a photonic-crystal micro-cavity. The linearly polarized emission of a selected quantumdot changes continuously its polarization angle, from nearly perpendicular to the cavity mode polarization at large detuning, to parallel at zero detuning, and reversing sign for negative detuning. The linear polarization rotation is qualitatively interpreted in terms of the detuning dependent mixing of the quantumdot and cavity states. The present result is relevant to achieve continuous control of the linear polarization in single photon emitters.

We report on the deterministic coupling between single semiconducting nanowire quantumdots emitting in the visible and plasmonic Au nanoantennas. Both systems are separately carefully characterized through microphotoluminescence and cathodoluminescence. A two-step realignment process using cathodoluminescence allows for electron beam lithography of Au antennas near individual nanowire quantumdots with a precision of 50 nm. A complete set of optical properties are measured before and after antenna fabrication. They evidence both an increase of the NW absorption, and an improvement of the quantumdot emission rate up to a factor two in presence of the antenna.

The influence of the electron-LO-phonon coupling on energy spectrum of the low -lying states of an exciton in parabolic quantumdots is investigated as a function of dot size.Calculations are made by using the method of few-body physics within the effective-mass approximation.A considerable decrease of the energy in the stronger confinement range is found for the low-lying states of an exction in quantumdots.Which results from the confinement of electron-phonon coupling.

Quantum-state engineering, i.e. active manipulation over the coherent dynamics of suitable quantum-mechanical systems, has become a fascinating prospect of modern physics. Here we discuss the dynamics of two interacting electrons in a coupledquantumdot driven by an external electric field. The results show that the two quantumdots can be used to prepare a maximally entangled Bell state by changing the strength and duration of an oscillatory electric field. Different from the suggestion made by Loss et al (1998 Phys. Rev. A 57 120), the present entanglement involves the spatial degree of freedom for the two electrons. We also find that the coherent tunnelling suppression discussed by Grossmann et al (1991 Phys. Rev. Lett. 67 516) persists in the two-particle case: i.e. two electrons initially localized in one dot can remain dynamically localized, although the strong Coulomb repulsion prevents them from behaving so. Surprisingly,the interaction enhances the degree of localization to a large extent compared with that in the non-interacting case.This phenomenon is referred to as the Coulomb-enhanced dynamical localization.

Experiments on semiconductor quantumdot systems have demonstrated the coupling between electron spins in quantumdots and spins localized in the neighboring area of the dots. Here we show that in a magnetic field the electrical current flowing through a single quantumdot tunnel-coupled to a spin displays a dip at the singlet-triplet anticrossing point which appears due to the spin-orbit interaction. We specify the requirements for which the current dip is formed and examine the properties of the dip for various system parameters, such as energy detuning, spin-orbit interaction strength, and coupling to leads. We suggest a parameter range in which the dip could be probed.

We study dynamics of the interaction between two weak light beams mediated by a strongly coupledquantumdot-photonic crystal cavity system. First, we perform all optical switching of a weak continuous-wave signal with a pulsed control beam, and then perform switching between two pulsed beams (40ps pulses) at the single photon level. Our results show that the quantumdot-nanocavity system creates strong, controllable interactions at the single photon level.

We demonstrate mid-infrared electroluminescence from intersublevel transitions in self-assembled InAs quantumdotscoupled to surface plasmon modes on metal hole arrays. Subwavelength metal hole arrays with different periodicity are patterned into the top contact of the broadband (9-15 {micro}m) quantumdot material and the measured electroluminescence is compared to devices without a metal hole array. The resulting normally directed emission is narrowed and a splitting in the spectral structure is observed. By applying a coupledquantum electrodynamic model and using reasonable values for quantumdot distributions and plasmon linewidths we are able to reproduce the experimentally measured spectral characteristics of device emission when using strong coupling parameters.

We show how the two interacting electrons in a field-driven coupledquantumdot can be used to prepare maximally entangled Bell states. The time durations of the oscillatory electric field for producing and maintaining such highly entangled states are identified by both analytic and exact numerical solutions of the quantum dynamical equations.

A spatially indirect exciton in vertically coupledquantumdots is considered with the use of 1/Q-expansion, where Q is the dimensionless quantum parameter determined by the ratio of characteristic Coulomb energy of electron-hole interaction to the energy of one-particle transition in a confining po

We report numerical simulations of biexciton generation in coupledquantumdots (CQDs) placed in a static electric field and excited by a chirped laser pulse. Our simulations explicitly account for exciton-phonon interactions at finite temperature using a non-Markovian quantum jump approach to solve

In self assembled III-V semiconductor quantumdots, valence holes have longer spin coherence times than the conduction electrons, due to their weaker coupling to nuclear spin bath fluctuations. Prolonging hole spin stability relies on a better understanding of the hole to nuclear spin hyperfine coupling which we address both in experiment and theory in the symmetric (111) GaAs/AlGaAs droplet dots. In magnetic fields applied along the growth axis, we create a strong nuclear spin polarization d...

Temporal group delays originating from the optical analogue to electromagnetically induced transparency (EIT) are compared in two systems. Similar transmission characteristics are observed between a coherently coupled high-Q multi-cavity array and a single quantumdot (QD) embedded cavity in the weak coupling regime. However, theoretically generated group delay values for the multi-cavity case are around two times higher. Both configurations allow direct scalability for chip-scale optical pulse trapping and coupled-cavity quantum electrodynamics (QED).

Strongly coupledquantumdot-cavity systems provide a non-linear configuration of hybridized light-matter states with promising quantum-optical applications. Here, we investigate the coherent interaction between strong laser pulses and quantumdot-cavity polaritons. Resonant excitation of polaritonic states and their interaction with phonons allow us to observe coherent Rabi oscillations and Ramsey fringes. Furthermore, we demonstrate complete coherent control of a quantumdot-photonic crystal cavity based quantum-bit. By controlling the excitation power and phase in a two-pulse excitation scheme we achieve access to the full Bloch sphere. Quantum-optical simulations are in good agreement with our experiments and provide insight into the decoherence mechanisms. PMID:27112420

We present a study of the strong coupling between radiation and matter, considering a system of two quantumdots, which are in mutual interaction and interact with a single mode of light confined in a semiconductor nanocavity. We take into account dissipative mechanisms such as the escape of the cavity photons, decay of the quantumdot excitons by spontaneous emission, and independent exciton pumping. It is shown that the mutual interaction between the dots can be measured off-resonance only if the strong coupling condition is reached. Using the quantum regression theorem, a reasonable definition of the dynamical coupling regimes is introduced in terms of the complex Rabi frequency. Finally, the emission spectrum for relevant conditions is presented and compared with the above definition, demonstrating that the interaction between the excitons does not affect the strong coupling.

Making use of the method of few-body physics, the energy spectrum of a four-electron system consisting in a vertically coupled double-layer quantumdot as a function of the strength ofa magnetic field is investigated. Discontinuous ground-state transitions induced by an external magnetic field are shown. We find that, in the strong coupling case, the ground-state transitions depend not only on the external magnetic field B but also on the distance d between double-layer quantumdots. However, in the case of weak coupling, the ground-state transitions occur in the new sequence of the values of the magic angular momentum. Hence, the interlayer separation d and electron-electron interaction strongly affect the ground state of the coupledquantumdots.

By employing a micrometer precision mechanical transfer technique, we embed individual InSb nanowires into a superconducting coplanar waveguide resonator. We investigate the characteristics of a double quantumdot formed in an InSb nanowire interacting with a single mode microwave field. The charge stability diagram can be obtained from the amplitude and phase response of the resonator independently from the dc transport measurement. As the charge transits between dot-dot, or dot-lead, the change of resonator transmission is compared and the charge-cavity coupling strength is extracted to be in the magnitude of several MHz.

By employing a micrometer precision mechanical transfer technique, we embed individual InSb nanowires into a superconducting coplanar waveguide resonator. We investigate the characteristics of a double quantumdot formed in an InSb nanowire interacting with a single mode microwave field. The charge stability diagram can be obtained from the amplitude and phase response of the resonator independently from the dc transport measurement. As the charge transits between dot-dot, or dot-lead, the change of resonator transmission is compared and the charge-cavity coupling strength is extracted to be in the magnitude of several MHz.

Full Text Available During the epitaxial growth method, self-assembled semi-spheroid-shaped quantumdots (QDs are formed on the wetting layer (WL. However for sake of simplicity, researchers sometimes assume semi-spheroid-shaped QDs to be dome-shaped (hemisphere. In this work, a detailed and comprehensive study on the difference between electronic and transition properties of dome- and semi-spheroid-shaped quantumdots is presented. We will explain why the P-to-S intersubband transition behaves the way it does. The calculated results for intersubband P-to-S transition properties of quantumdots show two different trends for dome-shaped and semi-spheroid-shaped quantumdots. The results are interpreted using the probability of finding electron inside the dome/spheroid region, with emphasis on the effects of wetting layer. It is shown that dome-shaped and semi-spheroid-shaped quantumdots feature different electronic and transition properties, arising from the difference in lateral dimensions between dome- and semi-spheroid-shaped QDs. Moreover, an analogy is presented between the bound S-states in the quantumdots and a simple 3D quantum mechanical particle in a box, and effective sizes are calculated. The results of this work will benefit researchers to present more realistic models of coupled QD/WL systems and explain their properties more precisely.

By means of sequential and cotunneling spectroscopy, we study the tunnel couplings between metallic leads and individual levels in a carbon nanotube quantumdot. The levels are ordered in shells consisting of two doublets with strong- and weak-tunnel couplings, leading to gate-dependent level...

Interfacing of single photon emitters, such as quantumdots, with photonic nanocavities enables study of fundamental quantum electrodynamic phenomena. In such experiments, the inability to precisely position quantum emitters at the nanoscale usually limits the ability to control spontaneous emission, despite sophisticated control of optical density of states by cavity design. Thus, effective light-matter interactions in photonic nanostructures strongly depend on deterministic positioning of quantum emitters. In this work by using directed self-assembly of DNA origami we demonstrate deterministic coupling of quantumdots with gallium phosphide (GaP) dielectric whispering gallery mode resonators design to enhance CdSe quantumdot emission at 600nm-650nm. GaP microdisk and microring resonators are dry-etched through 200nm layer of gallium phosphide on silicon dioxide/silicon substrates. Our simulations show that such GaP resonators may have quality factors up to 10^5, which ensures strong light-matter interaction. On the top surface of microresonators, we write binding sites in the shape of DNA origami using electron beam lithography, and use oxygen plasma exposure to chemically activate these binding sites. DNA origami self-assembly is accomplished by placing DNA origami - quantumdot complexes into these binding sites. This approach allows us to achieve deterministic placement of the quantumdots with a few nm precision in position relative to the resonator. We will report photoluminescence spectroscopy and lifetime measurements of quantumdot - resonator deterministic coupling to probe the cavity-enhanced spontaneous emission rate. Overall, this approach offers precise control of emitter positioning in nanophotonic structures, which is a critical step for scalable quantum information processing.

We present a molecular characterization of metal-affinity driven self-assembly between CdSe-ZnS quantumdots and a series of hexahistidine peptides with different charges. In particular, we uti- lized fluorescence coupled capillary electrophoresis to test the self-assembly process of quantumdots with peptides in solution. Four peptides with different charges can be efficiently separated by fluorescence coupled capillary electrophoresis. The migration time appeared to be influenced by the charges of the peptide. In addition, the kinetics of self-assembly process of quantumdots with one of the peptides manifested a bi-phasic kinetics followed by a saturating stage. This work revealed that there exist two types of binding sites on the surface of quantumdots for peptide 1: one type termed "high priority" binding site and a "low priority" site which is occupied after the first binding sites are fully occupied. The total self-assembly process finishes in solution within 80 s. Our work represents the systematic investigation of the details of self-assembly kinetics utilizing high-resolution fluorescence coupled capillary electrophoresis. The charge effect of peptide coating quantumdots provides a new way of preparing bioprobes.

We have studied the transport properties of a ring-coupledquantumdot array driven by an AC magnetic field,which is connected to two leads,and we give the response of the transport current to the dynamical localization. We found that when the ratio of the magnetic flux to the total quantumdots number is a root of the zeroth order Bessel function,dynamical localization and collapse of quasi-energy occurs and importantly,the transport current displays a dip which is the signal of dynamical localization. The dynamical localization effect is strengthened as a result of the increase of the quantumdot number,and it is weakened on account of the increase of the dots-lead hopping rate.

To lowest order in the coupling strength, the spin-orbit coupling in quantumdots results in a spin-dependent Aharonov-Bohm flux. This flux decouples the spin-up and spin-down random matrix theory ensembles of the quantumdot. We employ this ensemble and find significant changes in the distribution of the Coulomb blockade peak height, in particular, a decrease of the width of the distribution. The puzzling disagreement between standard random matrix theory and the experimental distributions by Patel et al. [Phys. Rev. Lett. 81, 5900 (1998)

We have studied the electronic structure of CdS/ZnSe coupledquantumdots, a novel heterostructure at the nanoscale. Our calculations reveal CdS/ZnSe coupledquantumdots are type II in nature where the anion p states play an important role in deciding the band offset for the highest occupied

We theoretically design a single-mode plasmonic ring nanocavity. Based on the plasmonic cavity, the exciton dynamics between two identical quantumdots (QD-p, QD-q) coupled to the nanocavity are investigated. It is shown that the coupling factors gi (i=p, q) between QD-i and surface plasmons are both equal to 12.53meV in our model and exciton population swap between the two QDs can be realized. The periods and amplitudes of population oscillations can be modified by the coupling factors. Our results may have potential applications in quantum information and quantum computation on a chip.

The band structure of bulk silicon has a sixfold valley degeneracy. Strain in the Si/SiGe quantum well system partially lifts the valley degeneracy, but the materials factors that set the splitting of the two lowest lying valleys are still under intense investigation. Using cavity input-output theory, we propose a method for accurately determining the valley splitting in Si/SiGe double quantumdots embedded in a superconducting microwave resonator. We show that low lying valley states in the double quantumdot energy level spectrum lead to readily observable features in the cavity transmission. These features generate a "fingerprint" of the microscopic energy level structure of a semiconductor double quantumdot, providing useful information on valley splittings and intervalley coupling rates.

Based on the variational method of Pekar type, we study the energies and the wave-functions of the ground and the first-excited states of magneto-bipolaron, which is strongly coupled to the LO phonon in a parabolic potential quantumdot under an applied magnetic field, thus built up a quantumdot magneto-bipolaron qubit. The results show that the oscillation period of the probability density of the two electrons in the qubit decreases with increasing electron-phonon coupling strength α, resonant frequency of the magnetic field ω c, confinement strength of the quantumdot ω 0, and dielectric constant ratio of the medium η the probability density of the two electrons in the qubit oscillates periodically with increasing time t, angular coordinate φ 2, and dielectric constant ratio of the medium η the probability of electron appearing near the center of the quantumdot is larger, and the probability of electron appearing away from the center of the quantumdot is much smaller. Project supported by the Natural Science Foundation of Hebei Province, China (Grant No. E2013407119) and the Items of Institution of Higher Education Scientific Research of Hebei Province and Inner Mongolia, China (Grant Nos. ZD20131008, Z2015149, Z2015219, and NJZY14189).

We investigate the quantum optical properties of a quantum-dot dipole emitter coupled to a finite-size metal nanoparticle using a photon Green-function technique that rigorously quantizes the electromagnetic fields. We first obtain pronounced Purcell factors and photonic Lamb shifts for both a 7......- and 20-nm-radius metal nanoparticle, without adopting a dipole approximation. We then consider a quantum-dot photon emitter positioned sufficiently near the metal nanoparticle so that the strong-coupling regime is possible. Accounting for nondipole interactions, quenching, and photon transport from...... the dot to the detector, we demonstrate that the strong-coupling regime should be observable in the far-field spontaneous emission spectrum, even at room temperature. The vacuum-induced emission spectra show that the usual vacuum Rabi doublet becomes a rich spectral triplet or quartet with two of the four...

@@ We theoretically study the properties of the ground state of the parallel-coupled double quantumdots embedded in a mesoscopic ring in the Kondo regime by means of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. Our results show that in this system, the persistent current depends sensitively on both the parity of this system and the size of the ring. Two dots can be coupled coherently, which is reflected in the giant current peak in the strong coupling regime. This system might be a candidate for future device applications.

We present theoretical and experimental results on the mechanical damping of an atomic force microscope cantilever strongly coupled to a self-assembled InAs quantumdot. When the cantilever oscillation amplitude is large, its motion dominates the charge dynamics of the dot which in turn leads to nonlinear, amplitude-dependent damping of the cantilever. We observe highly asymmetric line shapes of Coulomb blockade peaks in the damping that reflect the degeneracy of energy levels on the dot. Furthermore, we predict that excited state spectroscopy is possible by studying the damping versus oscillation amplitude, in analogy with varying the amplitude of an ac gate voltage.

The optical excitation of semiconductor gain media introduces both gain and refractive index changes, commonly referred to as amplitude-phase coupling. Quantum-confined structures with an energetically well separated carrier reservoir usually exhibit a decreased amplitude-phase coupling compared to bulk materials. However, its magnitude and definition is still controversially discussed. We investigate the fundamental processes influencing the amplitude-phase coupling in semiconductor quantum-dot media using a coupled-carrier rate-equation model. We are able to analyze the dependence on the electronic structure and suggest routes towards an optimization of the dynamic phase response of the gain material.

We present a theoretical study of the exciton (X0), the positive and negative trions (X+ and X-) in single and vertically coupled configurations of self-assembled InGaAs quantumdots under an electric field. The quantum states of X0, X+ and X- have been investigated using a quasi-one-dimensional (Q1D) model within the effective-mass approximation. For the single quantumdots, the electric-field dependent energy levels and the average inter-particle distances for the exciton and trions have been calculated. For the coupledquantumdots, the ground and the excited states for X0, X+ and X- have also been calculated and discussed. It is found that either the hole or the electron can be tuned into resonance states by the electric field and that the transition energy spectra for both trions consequently show crossing and anticrossing patterns. The recombination probabilities of the exciton and trion optical transitions are also calculated. The theoretical results have been compared with previously reported photoluminescence data and qualitative agreement is obtained. The trion conditional wave functions are also plotted under different electric field intensities, and it is found that a molecular orbital can be formed at a critical electric field intensity. The evolution of the energy levels of the trions in coupledquantumdots can be explained by the interplay of particle transfer and the electric field.

Considering Rashba quantum wires with a proximity-induced superconducting gap as physical realizations of Majorana bound states and quantumdots, we calculate the overlap of the Majorana wave functions with the local wave functions on the dot. We determine the spin-dependent tunneling amplitudes between these two localized states and show that we can tune into a fully spin polarized tunneling regime by changing the distance between dot and Majorana bound state. Upon directly applying this to the tunneling model Hamiltonian, we calculate the effective magnetic field on the quantumdot flanked by two Majorana bound states. The direction of the induced magnetic field on the dot depends on the occupation of the nonlocal fermion formed from the two Majorana end states which can be used as a readout for such a Majorana qubit.

We present measurements of the β-factor, describing the coupling efficiency of light emitted by single InAs/GaAs semiconductor quantumdots into a photonic crystal waveguide mode. The β-factor is evaluated by means of time resolved frequency-dependent photoluminescence spectroscopy. The emission...

We propose a scheme to efficiently couple a single quantumdot electron spin to an optical nano-cavity, which enables us to simultaneously benefit from a cavity as an efficient photonic interface, as well as to perform high fidelity (nearly 100%) spin initialization and manipulation achievable in...

For two coupledquantumdots within cavity QED, we show that the exciton-phonon interaction reduces the Rabi frequency and Foerster interaction even at absolute zero temperature. The exciton-phonon interaction also makes an additional contribution to the static exciton-exciton dipole interaction energy.

For two coupledquantumdots within cavity QED, we show that the exciton-phonon interaction reduces the Rabi frequency and Förster interaction even at absolute zero temperature. The exciton-phonon interaction also makes an additional contribution to the static exciton-exciton dipole interaction energy.

We demonstrate theoretically how photon-assisted dephasing by the electron-phonon interaction in a coupled cavity-quantumdot system can be significantly reduced for specific QD-cavity detunings. Our starting point is a recently published theory,1 which considers longitudinal acoustic phonons, de...

The effect of external noise, which is characterized by an Ornstein-Uhlenbeck process, on the dynamical localization of two coupling electrons in a quantumdot array under the action of an ac electric field is studied. A numerical solution of the stochastic equations is obtained by averaging over stochastic trajectories. The results show that the external noise may destroy the dynamical localization, but the anti-noise capacity of the system is stronger when the two electrons are localized at the ends of the quantumdot array.

Full Text Available Abstract We present transport measurements on a strongly coupled graphene quantumdot in a perpendicular magnetic field. The device consists of an etched single-layer graphene flake with two narrow constrictions separating a 140 nm diameter island from source and drain graphene contacts. Lateral graphene gates are used to electrostatically tune the device. Measurements of Coulomb resonances, including constriction resonances and Coulomb diamonds prove the functionality of the graphene quantumdot with a charging energy of approximately 4.5 meV. We show the evolution of Coulomb resonances as a function of perpendicular magnetic field, which provides indications of the formation of the graphene specific 0th Landau level. Finally, we demonstrate that the complex pattern superimposing the quantumdot energy spectra is due to the formation of additional localized states with increasing magnetic field.

The influence of the electron-phonon coupling on the energy of low-lying states of the barrier D- center,which consists of a positive ion located on the z-axis at a distance from the two-dimensional quantumdot plane and two electrons in the dot plane bound by the ion, is investigated at arbitrary strength of magnetic field by making use of the method of few-body physics. Discontinuous ground-state energy transitions induced by the magnetic field are reported.The dependence of the binding energy of the D- ground state on the quantumdot radius is obtained. A considerable enhancement of the binding is found for the D- ground state, which results from the confinement of electrons and electron-phonon coupling.

We theoretically study the properties of the ground state of a series-coupled double quantumdot embedded in a mesoscopic ring in the Kondo regime by means of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that two dots can be coupled coherently,which is reflected in the appearance of parity effects and the complex current-phase relation in this system. This system might be a possible candidate for future device applications.

We present measurements of a superconducting half-wave resonator coupled to two InSb nanowire quantumdots. Precise nanowire alignment at the electric field antinodes at opposite ends of the microwave cavity allows for a maximal electric field along the wire axis, without compromising the intrinsic quality factor of the cavity. This architecture may be useful for reaching the strong coupling limit between a single spin and a microwave photon, paving the way to on-chip coupling of single spins for quantum information processing.

We demonstrate the coexistence of lasing and strong coupling in a quantum-dot micropillar laser. Comprehensive experimental studies including measurements of the input-output curve, second- order photon-correlation and coherence time are used to identify the transition of a strongly coupledquantum-dot microcavity system to lasing. The experimental results are evaluated on the basis of a microscopic theory that includes contributions from detuned background emitters. Furthermore, we show that both the emission spectrum and the strong coupling condition are strongly modified at the laser threshold due to the higher-order photonic states required to reach lasing. By accounting for these states that become realized under strong pumping, we provide a closed analytic expression that describes the transition from strong to weak coupling across the threshold in agreement with both experiment and a numerical approach.

We explore the effects of asymmetry of hopping parameters between double parallel quantumdots and the leads on the conductance and a possibility of local magnetic moment formation in this system using functional renormalization group approach with the counterterm. We demonstrate a possibility of a quantum phase transition to a local moment regime [so-called singular Fermi liquid (SFL) state] for various types of hopping asymmetries and discuss respective gate voltage dependencies of the conductance. We show that, depending on the type of the asymmetry, the system can demonstrate either a first-order quantum phase transition to an SFL state, accompanied by a discontinuous change of the conductance, similarly to the symmetric case, or the second-order quantum phase transition, in which the conductance is continuous and exhibits Fano-type asymmetric resonance near the transition point. A semianalytical explanation of these different types of conductance behavior is presented.

We demonstrate a quantum-dot microcavity by coupling core-shell semiconductor nanocrystals to a fused-silica microsphere. We show that the composite microcavity can feature Q factors of the order of 10(8), providing a model system for investigating cavity QED and microlasers at the level of single quantumdots.

We propose to measure the differential conductance G as a function of the bias V for a quantumdot side-coupled to a topological superconductor to detect the existence of the chiral Majorana edge states. It turns out that G for the spinless dot is an oscillatory (but not periodic) function of eV due to the coupling to the chiral Majorana edge states, where -e is the charge carried by the electron. The behaviour of G versus eV is distinguished from that of a multi-level dot in three respects. First of all, due to the coupling to the topological superconductor, the value of G will shift upon adding or removing a vortex in the topological superconductor. Next, for an off-resonance dot, the conductance peak in the present case takes a universal value e(2)/(2h) when the two leads are symmetrically coupled to the dot. Finally, for a symmetric setup and an on-resonance dot, the conductance peak will approach the same universal value e(2)/(2h) at a large bias.

We theoretically investigate spin-dependent electron transport through an Aharonov-Bohm-Casher interferometer containing a laterally coupled double quantumdot. In particular, we numerically calculate the Aharonov-Bohm and Aharonov-Casher oscillations of the linear conductance in the Kondo regime. We show that the AC oscillation in the Kondo regime deviates from the sinusoidal form.

We analyze the energy spectrum of a neutral exciton confined in a semiconductor heterostructure formed by two vertically coupled axially symmetrical type II quantumdots located close to each other. The electron in the structure is mainly located inside dots tunneling between them while the hole generally is placed in the exterior region close to the symmetry axis. Solutions of the Schrödinger equation are obtained by a variational separation of variables in the adiabatic limit. Numerical results are presented for the energies of bonding and anti-bonding lowest-lying of the exciton states and for the density of states for different InP/GaInP quantumdots' morphologies and the magnetic field strength values.

Effects of a charged impurity on the ground state of two vertically coupled identical single-electron quantumdots with and without applied magnetic field are investigated. In the absence of the magnetic field, the investigations of the charged impurity effect on the quantum entanglement (QE) in some low-lying states are carried out. It is found that, both the positive charged impurity (PCI) and the negative charged impurity (NCI)reduce the QE in the low-lying states under consideration except that the QE in the ground state is enhanced by the NCI. Additionally, in the domain of B from 0 Tesla to 15 Tesla, the ground state energy E, the ground state angular momentum L and the ground state QE entropy S are worked out. As far as the ground state are concerned, the PCI (NCI) blocks (induces) the angular momentum phase transition and the QE phase transition besides the known fact (i. e., the PCI/NCI decreases/increases the energy) in the magnetic field.

In this theoretical work, we study a double quantumdot interacting strongly with a microcavity, while undergoing resonant tunneling. Effects of interdot tunneling on the light-matter hybridized states are determined, and tunability of their brightness degrees and associated dipole moments is demonstrated. These results predict dipolariton generation in artificial molecules coupled to optical resonators, and provide a promising scenario for control of emission efficiency and coherence times of exciton polaritons.

@@ We investigate the spin-flip process through double quantumdotscoupled to two ferromagnetic leads in series.By means of the slave-boson mean-field approximation, we calculate the density of states in the Kondo regime for two different configurations of the leads. It is found that transport shows some remarkable properties depending on the spin-flip strength. These effects may be useful in exploiting the role of electronic correlation in spintronics.

We propose a scheme to efficiently couple a single quantumdot electron spin to an optical nano-cavity, which enables us to simultaneously benefit from a cavity as an efficient photonic interface, as well as to perform high fidelity (nearly 100%) spin initialization and manipulation achievable in bulk semiconductors. Moreover, the presence of the cavity speeds up the spin initialization process beyond the GHz range.

Low lying states of a vertically coupled three-layer quantum-dot system are studied. Each layer contains oneelectron, and the tunnelling of electrons between layers is neglected. Effects of the interlayer separation d and theexternal magnetic field B are evaluated by numerical calculations. In the strong coupling case (i.e. d is small),as in a single dot, transitions of the angular momentum L of the true ground states occur when B increases,whereas in the weak coupling case the transition does not occur and L remains zero. Furthermore, it is foundthat the variation of d may also induce the L transition. As a result, a phase diagram of L of the true groundstate is given in the d - B plane.

We investigate the non-classical states of light that emerge in a microwave resonator coupled to a periodically-driven electron in a nanowire double quantumdot (DQD). Under certain drive configurations, we find that the resonator approaches a thermal state at the temperature of the surrounding substrate with a chemical potential given by a harmonic of the drive frequency. Away from these thermal regions we find regions of gain and loss, where the system can lase, or regions where the DQD acts as a single-photon source. These effects are observable in current devices and have broad utility for quantum optics with microwave photons. PMID:27517784

Motivated by recent experiments on the real-time single-electron counting through a semiconductor GaAs double quantumdot (DQD) by a nearby quantum point contact (QPC), we develop the full-counting statistics of coupled DQD and QPC system. By utilizing the time-scale separation between the dynamics of DQD and QPC, we derive the modified master equation with tunneling rates depending on the counting fields, which fulfill the detailed fluctuation theorem. Furthermore, we derive universal relations between the non-linear corrections to the current and noise, which can be verified in experiments.

Semiconductor quantumdots ("solid-state atoms") are promising candidates for quantum computers and future electronic and optoelectronic devices. Quantumdots are zero-dimensional electronic systems and therefore have discrete energy levels, similar to atoms or molecules. The size distribution...... of quantumdots, however, results in a large inhomogeneous broadening of quantumdot spectra. Work on self-assembled InGaAs/GaAs quantumdots will be presented. Properties of atom-like single-dot states are investigated optically using high spatial and spectral resolution. Single-dot spectra can be used...

Semiconductor quantumdots ("solid state atoms") are promising candidates for quantum computers and future electronic and optoelectronic devices. Quantumdots are zero-dimensional electronic systems and therefore have discrete energy levels, similar to atoms or molecules. The size distribution...... of quantumdots, however, results in a large inhomogeneous broadening of quantumdot spectra.Work on self-assembled InGaAs/GaAs quantumdots will be presented. Properties of atom-like single-dots states are investigated optically using high spatial and spectral resolution. Single-dot spectra can be used...

We present analytical results which describe the properties of the exciton ground state in a single semiconductor quantumdot (QD). Calculations are performed within the Luttinger-Kohn and Bir-Pikus Hamiltonian theory. We show in an explicit form that an interplay of the exchange interaction and the heavy hole-light hole coupling, which is due to the in-plane asymmetries of the dot shape and the strain distribution, plays an essential role. For both the bright and dark exciton, this combined effect leads to a dependence of the fine structure splitting and polarizations on the main anisotropy axis direction relative to the dot orientation. Basing on the obtained analytical expressions, we discuss some special cases in details.

The models presented simulate pumping techniques that can be used on modern semiconductor devices which are capable of coupling a quantumdot and cavity mode in order to determine a more efficient method of producing a single-photon emitter while taking into consideration typical parameters which are achievable given today’s standards of coupling strength. Cavity quantum electrodynamics are incorporated in the calculations as we compare various pumping schemes for the system that either use on-resonant laser excitation or nonresonant excitation due to a wetting layer. In particular, we look to study how antibunching effects change for each method as the cavity finesse is increased toward the strong coupling regime. Experimentally these studies are equivalent to nonlinear pump-probe measurements, where a strong pump, either resonant or nonresonant, is used to excite the coupled system, and the resulting state is characterized using a weak, resonant probe beam.

wavelength of single quantumdots is temperature tuned across the band edge of a photonic crystal waveguide and the spontaneous emission rate is recorded. Decay rates up to 5.7 ns−1, corresponding to a Purcell factor of 5.2, are measured and β-factors up to 85% are extracted. These results prove......We present measurements of the β-factor, describing the coupling efficiency of light emitted by single InAs/GaAs semiconductor quantumdots into a photonic crystal waveguide mode. The β-factor is evaluated by means of time resolved frequency-dependent photoluminescence spectroscopy. The emission...... the potential of photonic crystal waveguides in the realization of on-chip single-photon sources....

We model the electron and hole spin dynamics in an undoped double quantumdot structure, considering the carrier tunneling between quantumdots. Taking the presence of an additional in-plane or tilted magnetic field into account, we enable the simulation of magneto-optical experiments, like the time-resolved Kerr rotation measurement, which are currently performed on such structures to probe the temporal spin dynamics. With our model, we reproduce the experimentally observed effect of the extension of the spin polarization lifetime caused by spatial charge separation, which may occur in structures of this type. Moreover, we provide a number of qualitative predictions concerning the necessary conditions for observation of this effect as well as about possible channels of its suppression, including the spin–orbit coupling, which leads to tunneling of carriers accompanied by a spin flip. We also consider the impact of the magnetic field tilting, which results in an interesting spin polarization dynamics.

The effect of an ac electric field on quantum transport properties in a system of three quantumdots, two of which are connected in parallel, while the third is coupled to one of the other two, is investigated theoretically. Based on the Keldysh nonequilibrium Green's function method, the spin-dependent current, occupation number, and spin accumulation can be obtained in our model. An external magnetic flux, Rashba spin-orbit-coupling (SOC), and intradot Coulomb interactions are considered. The magnitude of the spin-dependent average current and the positions of the photon assisted tunneling (PAT) peaks can be accurately controlled and manipulated by simply varying the strength of the coupling and the frequency of the ac field. A particularly interesting result is the observation of a new kind of PAT peak and a multiple-PAT effect that can be generated and controlled by the coupling between the quantumdots. In addition, the spin occupation number and spin accumulation can be well controlled by the Rashba SOC and the magnetic flux.

By means of sequential and cotunneling spectroscopy, we study the tunnel couplings between metallic leads and individual levels in a carbon nanotube quantumdot. The levels are ordered in shells consisting of two doublets with strong- and weak-tunnel couplings, leading to gate-dependent level...... renormalization. By comparison to a one- and two-shell model, this is shown to be a consequence of disorder-induced valley mixing in the nanotube. Moreover, a parallel magnetic field is shown to reduce this mixing and thus suppress the effects of tunnel renormalization....

We study a quantumdot strongly coupled to a single high-finesse optical microcavity mode. We use a rotating wave approximation (RWA) method, commonly used in ion-laser interactions, together with the Lamb-Dicke approximation to obtain an analytic solution of this problem. The decay of Rabi oscillations because of the electron-phonon coupling is studied at arbitrary temperature and analytical expressions for the collapse and revival times are presented. Analyses without the RWA are presented as means of investigating the energy spectrum.

A coupledquantumdot-nanocavity system in the weak coupling regime of cavity-quantumelectrodynamics is dynamically tuned in and out of resonance by the coherent elastic field of a fSAW ≃ 800 MHz surface acoustic wave. When the system is brought to resonance by the sound wave, light-matter interaction is strongly increased by the Purcell effect. This leads to a precisely timed single photon emission as confirmed by the second order photon correlation function, g(2). All relevant frequencies of our experiment are faithfully identified in the Fourier transform of g(2), demonstrating high fidelity regulation of the stream of single photons emitted by the system.

We report on a template-stripping method for the direct surface patterning of colloidal quantum-dot thin films to produce highly luminescent structures with feature sizes less than 100 nm. Through the careful design of high quality bull’s-eye gratings we can produce strong directional beaming (10° divergence) with up to 6-fold out-coupling enhancement of spontaneous emission in the surface-normal direction. A transition to narrow single-mode lasing is observed in these same structures at thresholds as low as 120 μJ/cm2. In addition, we demonstrate that these structures can be fabricated on flexible substrates. Finally, making use of the size-tunable character of colloidal quantumdots, we demonstrate spectrally selective out-coupling of light from mixed quantum-dot films. Our results provide a straightforward route toward significantly improved optical properties of colloidal quantum-dot assemblies.

A coupledquantumdot--nanocavity system in the weak coupling regime of cavity quantumelectrodynamics is dynamically tuned in and out of resonance by the coherent elastic field of a $f_{\\rm SAW}\\simeq800\\,\\mathrm{MHz}$ surface acoustic wave. When the system is brought to resonance by the sound wave, light-matter interaction is strongly increased by the Purcell effect. This leads to a precisely timed single photon emission as confirmed by the second order photon correlation function $g^{(2)}$. All relevant frequencies of our experiment are faithfully identified in the Fourier transform of $g^{(2)}$, demonstrating high fidelity regulation of the stream of single photons emitted by the system. The implemented scheme can be directly extended to strongly coupled systems and acoustically drives non-adiabatic entangling quantum gates based on Landau-Zener transitions.

We conduct numerical simulations for an autonomous information engine comprising a set of coupled double quantumdots using a simple model. The steady-state entropy production rate in each component, heat and electron transfer rates are calculated via the probability distribution of the four electronic states from the master transition-rate equations. We define an information-engine efficiency based on the entropy change of the reservoir, implicating power generators that employ the environmental order as a new energy resource. We acquire device-design principles, toward the realization of corresponding practical energy converters, including that (1) higher energy levels of the detector-side reservoir than those of the detector dot provide significantly higher work production rates by faster states' circulation, (2) the efficiency is strongly dependent on the relative temperatures of the detector and system sides and becomes high in a particular Coulomb-interaction strength region between the quantumdots, and (3) the efficiency depends little on the system dot's energy level relative to its reservoir but largely on the antisymmetric relative amplitudes of the electronic tunneling rates.

Pairs of coupledquantumdots with controlled coupling between the two potential wells serve as an extremely rich system, exhibiting a plethora of optical phenomena that do not exist in each of the isolated constituent dots. Over the past decade, coupledquantum systems have been under extensive study in the context of epitaxially grown quantumdots (QDs), but only a handful of examples have been reported with colloidal QDs. This is mostly due to the difficulties in controllably growing nanoparticles that encapsulate within them two dots separated by an energetic barrier via colloidal synthesis methods. Recent advances in colloidal synthesis methods have enabled the first clear demonstrations of colloidal double quantumdots and allowed for the first exploratory studies into their optical properties. Nevertheless, colloidal double QDs can offer an extended level of structural manipulation that allows not only for a broader range of materials to be used as compared with epitaxially grown counterparts but also for more complex control over the coupling mechanisms and coupling strength between two spatially separated quantumdots. The photophysics of these nanostructures is governed by the balance between two coupling mechanisms. The first is via dipole-dipole interactions between the two constituent components, leading to energy transfer between them. The second is associated with overlap of excited carrier wave functions, leading to charge transfer and multicarrier interactions between the two components. The magnitude of the coupling between the two subcomponents is determined by the detailed potential landscape within the nanocrystals (NCs). One of the hallmarks of double QDs is the observation of dual-color emission from a single nanoparticle, which allows for detailed spectroscopy of their properties down to the single particle level. Furthermore, rational design of the two coupled subsystems enables one to tune the emission statistics from single photon

In self-assembled III-V semiconductor quantumdots, valence holes have longer spin coherence times than the conduction electrons, due to their weaker coupling to nuclear spin bath fluctuations. Prolonging hole spin stability relies on a better understanding of the hole to nuclear spin hyperfine coupling which we address both in experiment and theory in the symmetric (111) GaAs/AlGaAs droplet dots. In magnetic fields applied along the growth axis, we create a strong nuclear spin polarization detected through the positively charged trion X+ Zeeman and Overhauser splittings. The observation of four clearly resolved photoluminescence lines—a unique property of the (111) nanosystems—allows us to measure separately the electron and hole contribution to the Overhauser shift. The hyperfine interaction for holes is found to be about five times weaker than that for electrons. Our theory shows that this ratio depends not only on intrinsic material properties but also on the dot shape and carrier confinement through the heavy-hole mixing, an opportunity for engineering the hole-nuclear spin interaction by tuning dot size and shape.

Efficient and bright single photon sources at room temperature are critical components for quantum information systems such as quantum key distribution, quantum state teleportation, and quantum computation. However, the intrinsic radiative lifetime of quantum emitters is typically ∼10 ns, which severely limits the maximum single photon emission rate and thus entanglement rates. Here, we demonstrate the regime of ultrafast spontaneous emission (∼10 ps) from a single quantum emitter coupled to a plasmonic nanocavity at room temperature. The nanocavity integrated with a single colloidal semiconductor quantumdot produces a 540-fold decrease in the emission lifetime and a simultaneous 1900-fold increase in the total emission intensity. At the same time, the nanocavity acts as a highly efficient optical antenna directing the emission into a single lobe normal to the surface. This plasmonic platform is a versatile geometry into which a variety of other quantum emitters, such as crystal color centers, can be integrated for directional, room-temperature single photon emission rates exceeding 80 GHz.

We report on electronic transport measurements through a silicon double quantumdot consisting of a donor and a quantumdot. Transport spectra show resonant tunneling peaks involving different valley states, which illustrate the valley splitting in a quantumdot on a Si/SiO2 interface. The detailed gate bias dependence of double dot transport allows a first direct observation of the valley splitting in the quantumdot, which is controllable between 160 and 240 μeV with an electric field dependence 1.2 ± 0.2 meV/(MV/m). A large valley splitting is an essential requirement for implementing a physical electron spin qubit in a silicon quantumdot.

The electronic properties of laterally coupled InGaAs/GaAs quantumdot molecule are studied theoretically under in-plane electric field. The quantumdot molecule energy spectrum and envelope functions are calculated by solving one-band effective-mass Schrödinger equation with considering strain effect by employing finite element method in three dimensions. The obtained results indicate that the electron's coupling energy strongly depends on the In mole fractions of the nanohole, such that quantumdots act as a molecule in case that In mole fractions of dots are larger than that of nanohole. It can be also observed that electrons perceive the double-dots structure composing quantumdot molecule in close distances(less than 7 nm), while the holes discern two single dots structure. The effect of an in-plane electric field on the energy spectrum is investigated and it can be demonstrated that the coupling energy can be tuned by applying a low-intensity static electric field.

The spin-resolved thermoelectric transport properties of a quantumdotcoupled to ferromagnetic leads and side-coupled to a topological superconductor wire hosting Majorana zero-energy modes are studied theoretically. The calculations are performed in the linear response regime by using the numerical renormalization group method. It is shown that transport characteristics are determined by the interplay of Kondo correlations, exchange field due to the presence of ferromagnets and the strength of coupling to Majorana wire. These different energy scales are revealed in the behavior of the Seebeck and spin Seebeck coefficients, which exhibit an enhancement for temperatures of the order of the coupling strength to topological wire. Moreover, it is demonstrated that additional sign changes of the thermopower can occur due to the presence of Majorana zero-energy modes. These findings may provide additional fingerprints of the presence of Majorana fermions.

We investigate the spin-flip process through double quantumdotscoupled to two half-metallic ferromagnetic leads in series.By means of the slave-boson mean-field approximation,we calculate the density of states in the Kondo regime for two different configurations of the leads.It is found that the transport shows some remarkable properties depending on the spin-flip strength.These effects may be useful in exploiting the role of electronic correlation in spintronics.

We fabricated and studied a system comprising four site-controlled semiconductor quantumdots (QDs) embedded in a linear photonic crystal membrane cavity. The excellent position control and small spectral broadening permit coupling of the emission of all four QDs to the same photonic cavity modes. This is corroborated by co-polarization of the QD and cavity emission lines, as well as reduction in decay time, both with characteristic dependence on QD-cavity energy detuning. Scaling up to larger QD systems is discussed.

We theoretically study the spin-polarized transport phenomena of the parallel double quantumdotscoupled to two ferromagnetic leads by the Anderson Hamiltonian. The Hamiltonian is solved by means of the equation-of-motion approach. We analyse the transmission probability of this system in both the equilibrium and nonequilibrium cases, and our results reveal that the transport properties show some noticeable characteristics depending upon both the spin-polarized strength p and the value of the magnetic flux Ф. Moreover, in the parallel configuration, the position of the Kondo peak shifts while it remains unchanged for the antiparallel configuration. These effects might have some potential applications in spintronics.

We study theoretically the essential properties of an exciton in vertically coupled Gaussian quantumdots in the presence of an extemal magnetic field.The ground state energy of a heavy-hole exciton is split into four energy levels due to the Zeeman effect.For the symmetrical system,the entanglement entropy of the exciton state can reach a value of 1.However,for a system with broken symmetry,it is close to zero.Our results are in good agreement with previous studies.

We have investigated the persistent current in a mesoscopic ring with a side-coupledquantumdot. The problems are probed by using the one-impurity Anderson Hamiltonian and are treated with the slave boson mean field theory. It is shown that the persistent current in this system has the spin fluctuations, and the charge transfers between the two subsystems are suppressed in the limit of △/TKo < 1. The minimum value of the persistent current for ζK/L = 5 of the odd parity system provides an opportunity to detect the Kondo screening cloud.

Dynamical behaviors of an exciton in an asymmetric double coupledquantumdot and an altematingcurrent (ac) electric field have been analyzed based on the two-level approximation theory,and the conditions under which dynamical localization occurs are obtained.It shows that when the amplitude of the ac electric field is small,the Coulomb interaction plays an important role.The dynamical behaviors of the exciton are mainly confined in the low-level subspace.When the ratio of the field intensity to frequency is the root of Bessel function,electron and hole are localized in one dot,and they can be divided with the increasing amplitude of the ac electric field.

The conduction band states of GaAs-based vertically coupled double triangular quantumdots in two dimensions are investigated within the effective mass and parabolic approximation, using a diagonalization procedure to solve the corresponding Schrödinger-like equation. The effect of an externally applied static electric field is included in the calculation, and the variation of the lowest confined energy levels as a result of the change of the field strength is reported for different geometrical setups. The linear and nonlinear optical absorptions and the relative change of the refractive index, associated with the energy transition between the ground and the first excited state in the system, are studied as a function of the incident light frequency for distinct configurations of inter-dot distance and electric field intensities. The blueshift of the resonant absorption peaks is detected as a consequence of the increment in the field intensity, whereas the opposite effect is obtained from the increase of inter-dot vertical distance. It is also shown that for large enough values of the electric field there is a quenching of the optical absorption due to field-induced change of symmetry of the first excited state wavefunction, in the case of triangular dots of equal shape and size.

Strong coupling between semiconductor excitons and localized surface plasmons (LSPs) giving rise to hybridized plexciton states in which energy is coherently and reversibly exchanged between the components is vital, especially in the area of quantum information processing from fundamental and practical points of view. Here, in photoluminescence spectra, rather than from common extinction or reflection measurements, we report on the direct observation of Rabi splitting of approximately 160 meV as an indication of strong coupling between excited states of CdSe/ZnS quantumdots (QDs) and LSP modes of silver nanoshells under nonresonant nanosecond pulsed laser excitation at room temperature. The strong coupling manifests itself as an anticrossing-like behavior of the two newly formed polaritons when tuning the silver nanoshell plasmon energies across the exciton line of the QDs. Further analysis substantiates the essentiality of high pump energy and collective strong coupling of many QDs with the radiative dipole mode of the metallic nanoparticles for the realization of strong coupling. Our finding opens up interesting directions for the investigation of strong coupling between LSPs and excitons from the perspective of radiative recombination under easily accessible experimental conditions.

We study the thermoelectric coefficients of a multi-level quantumdot (QD) weakly coupled to two electron reservoirs in the Coulomb blockade regime. Detailed calculations and analytical expressions of the power factor and the figure of merit are presented. We restrict our interest to the limit where the energy separation between successive energy levels is much larger than the thermal energy (i.e., the quantum limit) and we report a giant enhancement of the figure of merit due to the violation of the Wiedemann-Franz law when phonons are frozen. We point out the similarity of the electronic and the phonon contribution to the thermal conductance for zero-dimensional electrons and phonons. Both contributions show an activated behavior. Our findings suggest that the control of the electron and phonon confinement effects can lead to nanostructures with improved thermoelectric properties.

The energy spectrum of a positively charged exciton confined in vertically coupled type II quantumdots with different morphologies in the presence of the external magnetic field is studied. The effect of the quantumdot morphology on the curves of the lowest energy levels as functions of the magnetic field is analyzed. It is shown that a strong correlation presented in this system generates the Aharonov–Bohm oscillations of the lower energy levels similar to those in wide quantum ring. The novel curves of the trion energies dependences on the external magnetic field for the disk-like, lens-like, and cone-like structures are presented.

Resonant coupling between distinct excitons in organic supramolecular assemblies and inorganic semiconductors is supposed to offer an approach to optoelectronic devices. Here, we report on colloidal nanohybrids consisting of self-assembled tubular J-aggregates decorated with semiconductor quantumdots (QDs) via electrostatic self-assembly. The role of QDs in the energy transfer process can be switched from a donor to an acceptor by tuning its size and thereby the excitonic transition energy while keeping the chemistry unaltered. QDs are located within a close distance (energy transfer coupling, which is around 92% in the case of energy transfer from the QD donor to the J-aggregate acceptor and approximately 20% in the reverse case. This system provides a model of an organic-inorganic light-harvesting complex using methods of self-assembly in aqueous solution, and it highlights a route toward hierarchical synthesis of structurally well-defined supramolecular objects with advanced functionality.

of quantumdots, however, results in a large inhomogeneous broadening of quantumdot spectra.Work on self-assembled InGaAs/GaAs quantumdots will be presented. Properties of atom-like single-dots states are investigated optically using high spatial and spectral resolution. Single-dot spectra can be used......Semiconductor quantumdots ("solid state atoms") are promising candidates for quantum computers and future electronic and optoelectronic devices. Quantumdots are zero-dimensional electronic systems and therefore have discrete energy levels, similar to atoms or molecules. The size distribution...... to probe coherence times of exciton states and relaxation processes, both of which are important for future applications....

of quantumdots, however, results in a large inhomogeneous broadening of quantumdot spectra. Work on self-assembled InGaAs/GaAs quantumdots will be presented. Properties of atom-like single-dot states are investigated optically using high spatial and spectral resolution. Single-dot spectra can be used......Semiconductor quantumdots ("solid-state atoms") are promising candidates for quantum computers and future electronic and optoelectronic devices. Quantumdots are zero-dimensional electronic systems and therefore have discrete energy levels, similar to atoms or molecules. The size distribution...... to probe coherence times of exciton states and relaxation processes, both of which are important for future applications....

We study the full counting statistics of transport electrons through a semiconductor two-level quantumdot with Rashba spin–orbit (SO) coupling, which acts as a nonabelian gauge field and thus induces the electron transition between two levels along with the spin flip. By means of the quantum master equation approach, shot noise and skewness are obtained at finite temperature with two-body Coulomb interaction. We particularly demonstrate the crucial effect of SO coupling on the super-Poissonian fluctuation of transport electrons, in terms of which the SO coupling can be probed by the zero-frequency cumulants. While the charge currents are not sensitive to the SO coupling.

We measured time-resolved photoluminescence from large quantumdots near a semiconductor-air interface. Far from the interface our data are consistent with dipole theory, but near the interface they question the validity of the dipole approximation.......We measured time-resolved photoluminescence from large quantumdots near a semiconductor-air interface. Far from the interface our data are consistent with dipole theory, but near the interface they question the validity of the dipole approximation....

Coupling nanoscale emitters via optical antennas enables comprehensive control of photon emission in terms of intensity, directivity and polarization. In this work we report highly directional emission of circularly polarized photons from quantumdotscoupled to a spiral optical antenna. The structural chirality of the spiral antenna imprints spin state to the emitted photons. Experimental results reveal that a circular polarization extinction ratio of 10 is obtainable. Furthermore, increasing the number of turns of the spiral gives rise to higher antenna gain and directivity, leading to higher field intensity and narrower angular width of emission pattern in the far field. For a five-turn Archimedes' spiral antenna, field intensity increase up to 70-fold simultaneously with antenna directivity of 11.7 dB has been measured in the experiment. The highly directional circularly polarized photon emission from such optically coupled spiral antenna may find important applications in single molecule sensing, quantum optics information processing and integrated photonic circuits as a nanoscale spin photon source.

We study the persistent currents induced by both the Aharonov-Bohm and Aharonov-Casher effects in a one-dimensional mesoscopic ring coupled to a side-branch quantumdot at Kondo resonance. For privileged values of the Aharonov-Bohm-Casher fluxes, the problem can be mapped onto an integrable model, exactly solvable by a Bethe ansatz. In the case of a pure magnetic Aharonov-Bohm flux, we find that the presence of the quantumdot has no effect on the persistent current. In contrast, the Kondo re...

The optical performance of a grating-coupled external cavity laser based on InAs/InP quantumdots is investigated. Continuous tuning from 1391 nm to 1468 nm is realized at an injection current of 1900 mA. With the injection current increasing to 2300 mA, the tuning is blue shifted to some extent to the range from 1383 nm to 1461 nm. By combining the effect of the injection current with the grating tuning, the total tuning bandwidth of the external cavity quantum-dot laser can reach up to 85 nm. The dependence of the threshold current on the tuning wavelength is also presented.

We investigate the joint effects of a magnetic field and electron-electron interaction on the tunneling density of states (DOS) of a quantumdotcoupled to the Luttinger liquid leads in the Kondo regime. We find that for intralead electron interaction, the DOS develops two peaks deviated from the origin by the Zeeman energy. With the increase of the intralead interaction, a phase transition occurs. For moderately strong interaction, the Zeeman splitting peaks develop into two dips. The splitting of the Kondo peak and dip is not symmetric with respect to up and down spins. In the limit of strong interaction the Zeeman splitting behavior disappears and there appears a power-law scaling behavior.

We present a semi-analytical model for studying the phase-amplitude coupling (α-factor) in quantumdot (QD) semiconductor lasers, which takes into account the influence of carrier populations in the excited state and in the two-dimensional carrier reservoir on the refractive index change. Calculations of the α-factor based on the amplified spontaneous emission method and on the “FM/AM” technique are both investigated. It is shown that the α-factor of a QD laser strongly depends on the energy separation between the ground state and the off-resonant states. Through band structure engineering, the α-factor can be reduced by enlarging this energy separation.

Full Text Available In this report, fluorescence detection coupled capillary electrophoresis (CE-FL was used to detect Protein A. Antibody was first labeled with Cy5 and then mixed with quantumdots (QDs to form QDs-antibody bioprobe. Further, we observed fluorescence resonance energy transfer (FRET from QDs donor to Cy5 acceptor. The bioprobe was formed and brought QDs and Cy5 close enough to allow FRET to occur. After adding protein A, the FRET system was broken and caused the FRET signal to decrease. Thus, a new method for the determination of protein A was proposed based on the FRET signal changes. This study provides a new trail of thought for the detection of protein.

Full Text Available Polaron states in cylindrical and spherical quantumdots with parabolic confinement potentials are investigated applying the Feynman variational principle. It is observed that for both kinds of quantumdots the polaron energy and mass increase with the increase of Frohlich electron-phonon coupling constant and confinement frequency. In the case of a spherical quantumdot, the polaron energy for the strong coupling is found to be greater than that of a cylindrical quantumdot. The energy and mass are found to be monotonically increasing functions of the coupling constant and the confinement frequency.

We demonstrate precise and quick detection of the positions of quantumdots (QDs) embedded in two-dimensional photonic crystal nanocavities. We apply this technique to investigate the QD position dependence of the optical coupling between the QD and the nanocavity. We use a scanning electron microscope (SEM) operating at a low acceleration voltage to detect surface bumps induced by the QDs buried underneath. This enables QD detection with a sub-10 nm precision. We then experimentally measure the vacuum Rabi spectra to extract the optical coupling strengths (gs) between single QDs and cavities, and compare them to the values estimated by a combination of the SEM-measured QD positions and electromagnetic cavity field simulations. We found a highly linear relationship between the local cavity field intensities and the QD-cavity gs, suggesting the validity of the point dipole approximation used in the estimation of the gs. The estimation using SEM has a small standard deviation of ±6.2%, which potentially enables the high accuracy prediction of g prior to optical measurements. Our technique will play a key role for deeply understanding the interaction between QDs and photonic nanostructures and for advancing QD-based cavity quantum electrodynamics.

We present the experimental realization of a quantum fiber-pigtail. The device consists of a semiconductor quantum-dot embedded into a conical photonic wire that is directly connected to the core of a fiber-pigtail. We demonstrate a photon collection efficiency at the output of the fiber of 5.8% ...

We describe an electrodynamic mechanism for coherent, quantum mechanical coupling between spacially separated quantumdots on a microchip. The technique is based on capacitive interactions between the electron charge and a superconducting transmission line resonator, and is closely related to atomic cavity quantum electrodynamics. We investigate several potential applications of this technique which have varying degrees of complexity. In particular, we demonstrate that this mechanism allows design and investigation of an on-chip double-dot microscopic maser. Moreover, the interaction may be extended to couple spatially separated electron spin states while only virtually populating fast-decaying superpositions of charge states. This represents an effective, controllable long-range interaction, which may facilitate implementation of quantum information processing with electron spin qubits and potentially allow coupling to other quantum systems such as atomic or superconducting qubits.

We present a rigorous medium-dependent theory for describing the quantum field emitted and detected from a single quantumdot exciton, strongly coupled to a planar photonic crystal nanocavity, from which the exact spectrum is derived. By using simple mode decomposition techniques, this exact spectrum is subsequently reduced to two separate user-friendly forms, in terms of the leaky cavity mode emission and the radiation mode emission. On application to study exciton-cavity coupling in the strong coupling regime, besides a pronounced modification of the usual vacuum Rabi spectral doublet, we predict several new effects associated with the leaky cavity mode emission, including the appearance of an off-resonance cavity mode and a loss-induced on-resonance spectral triplet. The cavity mode emission is shown to completely dominate the emitted spectrum, even for large cavity-exciton detunings, whereby the usual cavity-QED formulas developed for radiation-mode emission drastically fail. These predictions are in qualitative agreement with several "mystery observations" reported in recent experiments, and apply to a wide range of semiconductor cavities.

The light emission from reduced graphene oxide quantumdots (rGO-QDs) exhibit a significant enhancement in photoluminescence (PL) due to localized surface plasmon (LSP) interactions. Silver and gold nanoparticles (NPs) coupled to rGO nanoparticles exhibit the effect of resonant LSP coupling on the emission processes. Enhancement of the radiative recombination rate in the presence of Ag-NPs induced LSP tuned to the emission energy results in a four-fold increase in PL intensity. The localized field due to the resonantly coupled LSP modes induces n-π* transitions that are not observed in the absence of the resonant interaction of the plasmons with the excitons. An increase in the density of the Ag-NPs result in a detuning of the LSP energy from the emission energy of the nanoparticles. The detuning is due to the cumulative effect of the red-shift in the LSP energy and the electrostatic field induced blue shift in the PL energy of the rGO-QDs. The detuning quenches the PL emission from rGO-QDs at higher concentration of Ag NPs due to non-dissipative effects unlike plasmon induced Joule heating that occurs under resonance conditions. An increase in Au nanoparticles concentration results in an enhancement of PL emission due to electrostatic image charge effect.

The light emission from reduced graphene oxide quantumdots (rGO-QDs) exhibit a significant enhancement in photoluminescence (PL) due to localized surface plasmon (LSP) interactions. Silver and gold nanoparticles (NPs) coupled to rGO nanoparticles exhibit the effect of resonant LSP coupling on the emission processes. Enhancement of the radiative recombination rate in the presence of Ag-NPs induced LSP tuned to the emission energy results in a four-fold increase in PL intensity. The localized field due to the resonantly coupled LSP modes induces n-π* transitions that are not observed in the absence of the resonant interaction of the plasmons with the excitons. An increase in the density of the Ag-NPs result in a detuning of the LSP energy from the emission energy of the nanoparticles. The detuning is due to the cumulative effect of the red-shift in the LSP energy and the electrostatic field induced blue shift in the PL energy of the rGO-QDs. The detuning quenches the PL emission from rGO-QDs at higher concentration of Ag NPs due to non-dissipative effects unlike plasmon induced Joule heating that occurs under resonance conditions. An increase in Au nanoparticles concentration results in an enhancement of PL emission due to electrostatic image charge effect.

This review covers the description of the methodologies typically used for the calculation of the electronic structure of self-assembled and colloidal quantumdots. These are illustrated by the results of their application to a selected set of physical effects in quantumdots.

A four-terminal parallel double quantumdots (QDs) device is proposed to generate and detect the spin polarization in QDs. It is found that the spin accumulation in QDs and the spin-polarized currents in the upper and down leads can be generated when a bias voltage is applied between the left and right leads. It is more interesting that the spin polarization in the QDs can be detected using the upper and down leads. Moreover, the direction and magnitude of the spin polarization in the QDs, and in the upper and down leads can be tuned by the energy levels of QDs and the bias. -- Highlights: → The spin polarization in the quantumdots can be generated and controlled. → The spin polarization in quantumdots can be detected by the nonferromagnetic leads. → The system our studied is a discrete level spin Hall system.

Exciton and cavity mode (CM) dynamics in site-controlled pyramidal quantumdots (QDs), integrated with linear photonic crystal membrane cavities, are investigated for a range of temperatures and photo-excitation power levels. The absence of spurious multi-excitonic effects, normally observed in similar structures based on self-assembled QDs, permits the observation of effects intrinsic to two-level systems embedded in a solid state matrix and interacting with optical cavity modes. The coupled exciton and CM dynamics follow the same trend, indicating that the CM is fed only by the exciton transition. The Purcell reduction of the QD and CM decay times is reproduced well by a theoretical model that includes exciton linewidth broadening and temperature dependent non-radiative processes, from which we extract a Purcell factor of 17 ± 5. For excitation powers above QD saturation, we show the influence of quantum wire barrier states at short delay time, and demonstrate the absence of multiexcitonic background emission.

Understanding the cytotoxicity of quantumdots strongly relies upon the development of new analytical techniques to gather information about various aspects of the system. In this study, we demonstrate the in vivo biodistribution and fate of CdSe quantumdots in the murine model by means of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). By comparing the hot zones of each element acquired from LA-ICP-MS with those in fluorescence images, together with hematoxylin and eosin-stained images, we are able to perceive the fate and in vivo interactions between quantumdots and rat tissues. One hour after intravenous injection, we found that all of the quantumdots had been concentrated inside the spleen, liver and kidneys, while no quantumdots were found in other tissues (i.e., muscle, brain, lung, etc.). In the spleen, cadmium-114 signals always appeared in conjunction with iron signals, indicating that the quantumdots had been filtered from main vessels and then accumulated inside splenic red pulp. In the liver, the overlapped hot zones of quantumdots and those of phosphorus, copper, and zinc showed that these quantumdots have been retained inside hepatic cells. Importantly, it was noted that in the kidneys, quantumdots went into the cortical areas of adrenal glands. At the same time, hot zones of copper appeared in proximal tubules of the cortex. This could be a sign that the uptake of quantumdots initiates certain immune responses. Interestingly, the intensity of the selenium signals was not proportional to that of cadmium in all tissues. This could be the result of the decomposition of the quantumdots or matrix interference. In conclusion, the advantage in spatial resolution of LA-ICP-MS is one of the most powerful tools to probe the fate, interactions and biodistribution of quantumdots in vivo.

We review recent theoretical work on thermoelectric energy harvesting in multi-terminal quantum-dot setups. We first discuss several examples of nanoscale heat engines based on Coulomb-coupled conductors. In particular, we focus on quantumdots in the Coulomb-blockade regime, chaotic cavities and resonant tunneling through quantumdots and wells. We then turn toward quantum-dot heat engines that are driven by bosonic degrees of freedom such as phonons, magnons and microwave photons. These systems provide interesting connections to spin caloritronics and circuit quantum electrodynamics.

We demonstrate strong coupling between a surface plasmon and intersublevel transitions in self-assembled InAs quantumdots. The surface plasmon mode exists at the interface between the semiconductor emitter structure and a periodic array of holes perforating a metallic Pd/Ge/Au film that also serves as the top electrical contact for the emitters. Spectrally narrowed quantum-dot electroluminescence was observed for devices with varying subwavelength hole spacing. Devices designed for 9, 10, and 11 μm wavelength emission also exhibit a significant spectral splitting. The association of the splitting with quantum-dot Rabi oscillation is consistent with results from a calculation of spontaneous emission from an interacting plasmonic field and quantum-dot ensemble. The fact that this Rabi oscillation can be observed in an incoherently excited, highly inhomogeneously broadened system demonstrates the utility of intersublevel transitions in quantumdots for investigations of coherent transient and quantum coherence phenomena.

We have been investigating the synthesis of quantumdots of CdSe, CuInS2, and CuInSe2 for use in an intermediate bandgap solar cell. We have prepared a variety of quantumdots using the typical organometallic synthesis routes pioneered by Bawendi, et. al., in the early 1990's. However, unlike previous work in this area we have also utilized single-source precursor molecules in the synthesis process. We will present XRD, TEM, SEM and EDS characterization of our initial attempts at fabricating these quantumdots. Investigation of the size distributions of these nanoparticles via laser light scattering and scanning electron microscopy will be presented. Theoretical estimates on appropriate quantumdot composition, size, and inter-dot spacing along with potential scenarios for solar cell fabrication will be discussed.

Two methods, coupling self-assembled monolayer and chemical bath deposition (CBD), were utilized to assemble cadmium sulfide (CdS) quantumdots (QDs) onto mesoporous TiO2 films for dye-sensitized solar cell (DSSC) applications. Colloidal CdS QDs were first self-assembled on the TiO2 surface. CBD was then introduced to replenish the incorporated amount and increase the coverage ratio of CdS QDs on the TiO2 surface. The preassembled CdS QDs act as nucleation sites in the CBD process, forming a CdS nanofilm with an interfacial structure capable of inhibiting the recombination of injected electrons. An efficiency as high as 1.35% for the QD-sensitized DSSC was achieved using the present strategy.

We investigate the quantum transport properties through a special kind of quantumdot (QD) system composed of a serially coupled multi-QD-pair (multi-QDP) chain and side-coupled Majorana bound states (MBSs) by using the Green functions method, where the conductance can be classified into two kinds: the electron tunneling (ET) conductance and the Andreev reflection (AR) one. First we find that for the nonzero MBS-QDP coupling a sharp AR-induced zero-bias conductance peak with the height of e 2/h is present (or absent) when the MBS is coupled to the far left (or the other) QDP. Moreover, the MBS-QDP coupling can suppress the ET conductance and strengthen the AR one, and further split into two sub-peaks each of the total conductance peaks of the isolated multi-QDPs, indicating that the MBS will make obvious influences on the competition between the ET and AR processes. Then we find that the tunneling rate Γ L is able to affect the conductances of leads L and R in different ways, demonstrating that there exists a Γ L-related competition between the AR and ET processes. Finally we consider the effect of the inter-MBS coupling on the conductances of the multi-QDP chains and it is shown that the inter-MBS coupling will split the zero-bias conductance peak with the height of e 2/h into two sub-peaks. As the inter-MBS coupling becomes stronger, the two sub-peaks are pushed away from each other and simultaneously become lower, which is opposite to that of the single QDP chain where the two sub-peaks with the height of about e 2/2h become higher. Also, the decay of the conductance sub-peaks with the increase of the MBS-QDP coupling becomes slower as the number of the QDPs becomes larger. This research should be an important extension in studying the transport properties in the kind of QD systems coupled with the side MBSs, which is helpful for understanding the nature of the MBSs, as well as the MBS-related QD transport properties. Project supported by the National Natural

We study the persistent currents induced by both the Aharonov-Bohm and Aharonov-Casher effects in a one-dimensional mesoscopic ring coupled to a sidebranch quantumdot at Kondo resonance. For privileged values of the Aharonov-Bohm-Casher fluxes, the problem can be mapped onto an integrable model, exactly solvable by a Bethe ansatz. In the case of a pure magnetic Aharonov-Bohm flux, we find that the presence of the quantumdot has no effect on the persistent current. In contrast, the Kondo resonance interferes with the spin-dependent Aharonov-Casher effect to induce a current which, in the strong-coupling limit, is independent of the number of electrons in the ring.

We study the persistent currents induced by both the Aharonov-Bohm and Aharonov-Casher effects in a one-dimensional mesoscopic ring coupled to a sidebranch quantumdot at Kondo resonance. For privileged values of the Aharonov-Bohm-Casher fluxes, the problem can be mapped onto an integrable model, exactly solvable by a Bethe ansatz. In the case of a pure magnetic Aharonov-Bohm flux, we find that the presence of the quantumdot has no effect on the persistent current. In contrast, the Kondo resonance interferes with the spin-dependent Aharonov-Casher effect to induce a current which, in the strong-coupling limit, is independent of the number of electrons in the ring.

We theoretically study the properties of the ground state of the parallel-coupled double quantumdots embedded in a mesoscopic ring in the Kondo regime by means of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. We find that in this system, the persistent current depends sensitively on both the parity of this system and the size of the ring. In the strong coupling regime, the giant sharp current peak appears, at the same time, the parity dependence of the persistent current disappears. These imply that in the strong coupling regime, there exists giant Kondo resonance and the two dots can be coupled coherently. Thus this system might be a candidate for future device applications.

On the basis of the scattering matrix theory and nonequilibrium green function method, we have investigated the fluctuations of charge and spin current of the systems that consist of a quantumdot (QD) with a resonant level coupled to two semiconductor contacts within in alternative site (AS) and alternative bond (AB) framework, where two transverse (Bx) and longitudinal (Bz) magnetic fields are applied to the QD. It is only necessary to use the autocorrelation function to characterize the fluctuations of charge current for a twoterminal system because of the relation that is defined as Σα e Sαβ = Σβ e Sαβ = 0. Our result shows that both auto-shot noise (SLL) and cross-shot noise (SLR) are essential to characterize the fluctuations of spin current when Bx is present. Moreover, our model calculations show that the sign of the cross-shot noise of spin current is negative for all surface states of AS/QD/AS junctions, whereas it oscillates between positive and negative values for two surface states of AB/QD/AB junctions as we sweep the gate voltage.

Full Text Available Mycobacteria have always proven difficult to identify due to their low growth rate and fastidious nature. Therefore molecular biology and more recently nanotechnology, have been exploited from early on for the detection of these pathogens. Here we present the first stage of development of an assay incorporating cadmium selenide quantumdots (QDs for the detection of mycobacterial surface antigens. The principle of the assay is the separation of bacterial cells using magnetic beads coupled with genus-specific polyclonal antibodies and monoclonal antibodies for heparin-binding hemagglutinin. These complexes are then tagged with anti-mouse biotinylated antibody and finally streptavidin-conjugated QDs which leads to the detection of a fluorescent signal. For the evaluation of performance, the method under study was applied on Mycobacterium bovis BCG and Mycobacterium tuberculosis (positive controls, as well as E. coli and Salmonella spp. that constituted the negative controls. The direct observation of the latter category of samples did not reveal fluorescence as opposed to the mycobacteria mentioned above. The minimum detection limit of the assay was defined to 10(4 bacteria/ml, which could be further decreased by a 1 log when fluorescence was measured with a spectrofluorometer. The method described here can be easily adjusted for any other protein target of either the pathogen or the host, and once fully developed it will be directly applicable on clinical samples.

We investigate electron transport inside a ring system composed of a quantumdot (QD) coupled to two Majorana bound states confined at the ends of a one-dimensional topological superconductor nanowire. By tuning the magnetic flux threading through the ring, the model system we consider can be switched into states with or without zero-energy modes when the nanowire is in its topological phase. We find that the Fano profile in the conductance spectrum due to the interference between bound and continuum states exhibits markedly different features for these two different situations, which consequently can be used to detect the Majorana zero-energy mode. Most interestingly, as a periodic function of magnetic flux, the conductance shows 2 π periodicity when the two Majorana bound states are nonoverlapping (as in an infinitely long nanowire) but displays 4 π periodicity when the overlapping becomes nonzero (as in a finite length nanowire). We map the model system into a QD-Kitaev ring in the Majorana fermion representation and affirm these different characteristics by checking the energy spectrum.

Quantum logic gates are the key elements in quantum computing. Here we investigate the possibility of achieving a scalable and compact quantum computing based on stationary electron-spin qubits, by using the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics. We design the compact quantum circuits for implementing universal and deterministic quantum gates for electron-spin systems, including the two-qubit CNOT gate and the three-qubit Toffoli gate. They are compact and economic, and they do not require additional electron-spin qubits. Moreover, our devices have good scalability and are attractive as they both are based on solid-state quantum systems and the qubits are stationary. They are feasible with the current experimental technology, and both high fidelity and high efficiency can be achieved when the ratio of the side leakage to the cavity decay is low.

This book reviews recent advances in the exciting and rapidly growing field of quantumdot molecules (QDMs). It offers state-of-the-art coverage of novel techniques and connects fundamental physical properties with device design.

This book reflects the current status of theoretical and experimental research of graphene based nanostructures, in particular quantumdots, at a level accessible to young researchers, graduate students, experimentalists and theorists. It presents the current state of research of graphene quantumdots, a single or few monolayer thick islands of graphene. It introduces the reader to the electronic and optical properties of graphite, intercalated graphite and graphene, including Dirac fermions, Berry's phase associated with sublattices and valley degeneracy, covers single particle properties of

It have been recognized that the coupling of graphene quantumdots (GQDs) with semiconductor photocatalysts endow the resulting nanocomposites with enhanced photocatalytic performances, however, the essential roles of GQDs have not been clearly revealed yet. Herein, we report that a high efficiency of the photocatalytic H2 evolution was achieved using strongly coupled nanohybrids of CdS with GQDs (CdS/GQDs) as visible-light-driven photocatalysts. CdS/GQDs nanohybrids were synthesized by a facile hydrothermal method in which the crystallization of CdS precursor and coupling of GQDs could be accomplished in one-step. GQDs are firmly decorated on the surface of CdS nanoparticles, forming “dot-on-particle” heterodimer structures. GQDs have no significant influence on the crystallite structure of CdS but render the nanohybrids with strong light absorption at the wavelength beyond the band edge of CdS. Under visible light irradiation (≥420nm), CdS/GQDs nanohybrids reach the highest H2 production rate of 95.4μmol·h−1, about 2.7 times higher than that of pure CdS nanoparticles, at GQDs content of 1.0wt %, and the apparent quantum efficiency (AQE) was determined to be 4.2% at 420nm. Incident light-wavelength dependent experiments reveal that the light absorption of CdS dominated the performance of nanohybrids, and the excess light absorption coming from GQDs hardly contributes to the observed higher activity. Photocurrent response, steady-state and time-resolved PL, and EIS measurements suggest that the high activity of CdS/GQDs is attributed predominantly to the graphene-like nature of GQDs, which can act as an efficient electron acceptor to induce an efficient charge separation. This work clearly reveals that GQDs mainly played a role of electron acceptor instead of a photosensitizer in enhancing the photocatalytic H2 evolution performances of CdS/GQDs nanohybrids, which offers a new insight to understand the essential roles of GQDs in semiconductor

Based on the Huybrechts＇ linear-combination operator, effects of thermal lattice vibration on the effective potential of weak-coupling bipolaron in semiconductor quantumdots are studied by using the LLP variational method and quantum statistical theory. The results show that the absolute value of the induced potential of the bipolaron increases with increasing the electron-phonon coupling strength, but decreases with increasing the temperature and the distance of electrons, respectively; the absolute value of the effective potential increases with increasing the radius of the quantumdot, electron-phonon coupling strength and the distance of electrons, respectively, but decreases with increasing the temperature; the temperature and electron-phonon interaction have the important influence on the formation and state properties of the bipolaron： the bipolarons in the bound state are closer and more stable when the electron-phonon coupling strength is larger or the temperature is lower; the confinement potential and coulomb repulsive potential between electrons are unfavorable to the formation of bipolarons in the bound state.

Aharonov-Bohm (AB) oscillations are studied for a parallel-coupled vertical double quantumdot with a common source and drain electrode. We observe AB oscillations of current via a one-electron bonding state as the ground state and an antibonding state as the excited state. As the center gate voltage becomes more negative, the oscillation period is clearly halved for both the bonding and antibonding states, and the phase changes by half a period for the antibonding state. This result can be explained by a calculation that takes account of the indirect interdot coupling via the two electrodes.

We systematically study the influence of ferromagnetic leads on the Kondo resonance in a quantumdot tuned to the local moment regime. We employ Wilson’s numerical renormalization group method, extended to handle leads with a spin asymmetric density of states, to identify the effects of (i) a finite spin polarization in the leads (at the Fermi surface), (ii) a Stoner splitting in the bands (governed by the band edges), and (iii) an arbitrary shape of the lead density of states. For a generic lead density of states, the quantumdot favors being occupied by a particular spin species due to exchange interaction with ferromagnetic leads, leading to suppression and splitting of the Kondo resonance. The application of a magnetic field can compensate this asymmetry, restoring the Kondo effect. We study both the gate voltage dependence (for a fixed band structure in the leads) and the spin polarization dependence (for fixed gate voltage) of this compensation field for various types of bands. Interestingly, we find that the full recovery of the Kondo resonance of a quantumdot in the presence of leads with an energy-dependent density of states is possible not only by an appropriately tuned external magnetic field but also via an appropriately tuned gate voltage. For flat bands, simple formulas for the splitting of the local level as a function of the spin polarization and gate voltage are given.

The third generation of solar cells includes those based on semiconductor quantumdots. This sophisticated technology applies nanotechnology and quantum mechanics theory to enhance the performance of ordinary solar cells. Although a practical application of quantumdot solar cells has yet to be achieved, a large number of theoretical calculations and experimental studies have confirmed the potential for meeting the requirement for ultra-high conversion efficiency. In this book, high-profile scientists have contributed tutorial chapters that outline the methods used in and the results of variou

Harnessing the energy transfer interactions between the optical protein bacteriorhodopsin (bR) and CdSe/ZnS quantumdots (QDs) could provide a novel bio-nano electronics substrate with a variety of applications. In the present study, a polydimethyldiallyammonium chloride based I-SAM technique has been utilized to produce bilayers, trilayers and multilayers of alternating monolayers of bR, PDAC and QD's on a conductive ITO substrate. The construction of multilayer systems was directly monitored by measuring the unique A570 nm absorbance of bR, as well as QD fluorescence emission. Both of these parameters displayed a linear relationship to the number of monolayers present on the ITO substrate. The photovoltaic response of bilayers of bR/PDAC was observed over a range of 3 to 12 bilayers and the ability to efficiently create an electrically active multilayered substrate composed of bR and QDs has been demonstrated for the first time. Evaluation of QD fluorescence emission in the multilayer system strongly suggests that FRET coupling is occurring and, since the I-SAM technique provide a means to control the bR/QD separation distance on the nanometer scale, this technique may prove highly valuable for optimizing the distance dependent energy transfer effects for maximum sensitivity to target molecule binding by a biosensor. Finally, preliminary studies on the production of a sensor protein/bR hybrid gene construct are presented. It is proposed that the energy associated with target molecule binding to a hybrid sensor protein would provide a means to directly modulate the electrical output from a sensor protein/bR biosensor platform.

Cellular imaging has emerged as an important tool to unravel biological complexity and to accelerate the drug-discovery process, including cell-based screening, target identification, and mechanism of action studies. Recently, semiconductor nanoparticles known as quantumdots (QDs) have attracted great interest in cellular imaging applications due to their unique photophysical properties such as size, tunable optical property, multiplexing capability, and photostability. Herein, we show that QDs can also be applied to assay development and eventually to high-throughput/content screening (HTS/HCS) for drug discovery. We have synthesized QDs modified with PEG and primary antibodies to be used as fluorescent probes for a cell-based HTS system. The G protein-coupled receptor (GPCR) family is known to be involved in most major diseases. We therefore constructed human osteosarcoma (U2OS) cells that specifically overexpress two types of differently tagged GPCRs: influenza hemagglutinin (HA) peptide-tagged κ-opioid receptors (κ-ORs) and GFP-tagged A3 adenosine receptors (A3AR). In this study, we have demonstrated that 1) anti-HA antibody-conjugated QDs could specifically label HA-tagged κ-ORs, 2) subsequent treatment of QD-tagged GPCR agonists allowed agonist-induced translocation to be monitored in real time, 3) excellent emission spectral properties of QD permitted the simultaneous detection of two GPCRs in one cell, and 4) the robust imaging capabilities of the QD-antibody conjugates could lead to reproducible quantitative data from high-content cellular images. These results suggest that the present QD-based GPCR inhibitor screening system can be a promising platform for further drug screening applications.

Materials optimized for single-junction solar spectral harvesting, such as silicon, perovskites, and large-band-gap colloidal quantumdot solids, fail to absorb the considerable infrared spectral energy that lies below their respective band gap. Here we explore through modeling and experiment the potential for colloidal quantumdots (CQDs) to augment the performance of solar cells by harnessing transmitted light in the infrared. Through detailed balance modeling, we identify the CQD band gap that is best able to augment wafer-based, thin-film, and also solution-processed photovoltaic (PV) materials. The required quantumdots, with an excitonic peak at 1.3 μm, have not previously been studied in depth for solar performance. Using computational studies we find that a new ligand scheme distinct from that employed in better-explored 0.95 μm band gap PbS CQDs is necessary; only via the solution-phase application of a short bromothiol can we prevent dot fusion during ensuing solid-state film treatments and simultaneously offer a high valence band-edge density of states to enhance hole transport. Photoluminescence spectra and transient studies confirm the desired narrowed emission peaks and reduced surface-trap-associated decay. Electronic characterization reveals that only through the use of the bromothiol ligands is strong hole transport retained. The films, when used to make PV devices, achieve the highest AM1.5 power conversion efficiency yet reported in a solution-processed material having a sub-1 eV band gap.

The optical properties of the Stranski-Krastanov (S-K) grown InAs/GaAsSb quantumdots (QDs) coupled to sub-monolayer (SML) InAs QD stacks are investigated using photoluminescence (PL) spectroscopy. The PL emission peak of the S-K QDs shifts to shorter wavelengths with increasing the number of SML stacks (NSML) due to the increasing strain fields from the SML QDs. The PL peak energy is linearly increased with increasing the cube root of excitation power, with a different ratio of the absorption coefficient to radiative recombination rate for all the QD samples. The total carrier lifetime for the S-K QDs is increased with increasing NSML, most probably caused by the increase in the ground-state transition energy of the S-K QDs. The nonmonotonic behavior of the thermal activation energy of electrons in the S-K QDs is observed due to the NSML-dependent variation of the strain and Coulombic interaction within the QDs.

Hybrid nanowires combining semiconductor and superconductor materials appear well suited for the creation, detection, and control of Majorana bound states (MBSs). We demonstrate the emergence of MBSs from coalescing Andreev bound states (ABSs) in a hybrid InAs nanowire with epitaxial Al, using a quantumdot at the end of the nanowire as a spectrometer. Electrostatic gating tuned the nanowire density to a regime of one or a few ABSs. In an applied axial magnetic field, a topological phase emerges in which ABSs move to zero energy and remain there, forming MBSs. We observed hybridization of the MBS with the end-dot bound state, which is in agreement with a numerical model. The ABS/MBS spectra provide parameters that are useful for understanding topological superconductivity in this system.

Full Text Available The strong coupling limit of cavity quantum electrodynamics (QED implies the capability of a matterlike quantum system to coherently transform an individual excitation into a single photon within a resonant structure. This not only enables essential processes required for quantum information processing but also allows for fundamental studies of matter-light interaction. In this work, we demonstrate strong coupling between the charge degree of freedom in a gate-defined GaAs double quantumdot (DQD and a frequency-tunable high impedance resonator realized using an array of superconducting quantum interference devices. In the resonant regime, we resolve the vacuum Rabi mode splitting of size 2g/2π=238 MHz at a resonator linewidth κ/2π=12 MHz and a DQD charge qubit decoherence rate of γ_{2}/2π=40 MHz extracted independently from microwave spectroscopy in the dispersive regime. Our measurements indicate a viable path towards using circuit-based cavity QED for quantum information processing in semiconductor nanostructures.

We study hexagonal graphene quantumdots, using density functional theory, to obtain a quantitative description of the electronic properties and their size dependence, considering disk and ring geometries with both armchair and zigzag edges. We show that the electronic properties of quantumdots with armchair edges are more sensitive to structural details than those with zigzag edges. As functions of the inner and outer radii, we find in the case of armchair edges that the size of the band gap follows distinct branches, while in the case of zigzag edges it changes monotonically. This behaviour is further analyzed by studying the ground state wave function and explained in terms of its localisation.

Many photonic quantum information processing applications would benefit from a high brightness, fiber-coupled source of triggered single photons. Here, we present a fiber-coupled photonic-crystal waveguide single-photon source relying on evanescent coupling of the light field from a tapered out-coupler to an optical fiber. A two-step approach is taken where the performance of the tapered out-coupler is recorded first on an independent device containing an on-chip reflector. Reflection measurements establish that the chip-to-fiber coupling efficiency exceeds 80 %. The detailed characterization of a high-efficiency photonic-crystal waveguide extended with a tapered out-coupling section is then performed. The corresponding overall single-photon source efficiency is 10.9 % $\\pm$ 2.3 %, which quantifies the success probability to prepare an exciton in the quantumdot, couple it out as a photon in the waveguide, and subsequently transfer it to the fiber. The applied out-coupling method is robust, stable over time, ...

We present investigations of the propagation length of guided surface plasmon polaritons along Au waveguides on GaAs and their coupling to near surface InGaAs self-assembled quantumdots. Our results reveal surface plasmon propagation lengths ranging from 13.4 {\\pm} 1.7 {\\mu}m to 27.5 {\\pm} 1.5 {\\mu}m as the width of the waveguide increases from 2-5 {\\mu}m. Experiments performed on active structures containing near surface quantumdots clearly show that the propagating plasmon mode excites the dot, providing a new method to spatially image the surface plasmon mode. We use low temperature confocal microscopy with polarization control in the excitation and detection channel. After excitation, plasmons propagate along the waveguide and are scattered into the far field at the end. By comparing length and width evolution of the waveguide losses we determine the plasmon propagation length to be 27.5 {\\pm} 1.5 {\\mu}m at 830 nm (for a width of 5 {\\mu}m), reducing to 13.4 {\\pm} 1.7 {\\mu}m for a width of 2 {\\mu}m. For ...

Chiral molecules made of coupled achiral semiconductor nanocrystals, also known as quantumdots, show great promise for photonic applications owing to their prospective uses as configurable building blocks for optically active structures, materials, and devices. Here we present a simple model of optically active quantum-dot molecules, in which each of the quantumdots is assigned a dipole moment associated with the fundamental interband transition between the size-quantized states of its confined charge carriers. This model is used to analytically calculate the rotatory strengths of optical transitions occurring upon the excitation of chiral dimers, trimers, and tetramers of general configurations. The rotatory strengths of such quantum-dot molecules are found to exceed the typical rotatory strengths of chiral molecules by five to six orders of magnitude. We also study how the optical activity of quantum-dot molecules shows up in their circular dichroism spectra when the energy gap between the molecular states is much smaller than the states' lifetime, and maximize the strengths of the circular dichroism peaks by optimizing orientations of the quantumdots in the molecules. Our analytical results provide clear design guidelines for quantum-dot molecules and can prove useful in engineering optically active quantum-dot supercrystals and photonic devices.

We demonstrate the formation and spatial modulation of strongly coupled gallium selenide quantumdot (QD) nanoassemblies suspended in a nematic liquid-crystal (NLC) matrix at room temperature. Using static and dynamic optical techniques we show that the coupled QDs aggregate with a well-defined directionality commensurate with the NLC director axis. This results in highly anisotropic spectral properties of the QD assembly. The spatial orientation of the aggregates is selectively controlled in situ by the application of in-plane electric fields. The strong interdot coupling further increases the excitonic recombination rate which is both direction and electric field dependent. This electrical modulation, a noninvasive process, could potentially be an important functionality for the design and creation of building blocks for novel optoelectronic devices.

We report on strong mode coupling in closely spaced GaInP microdisk dimer structures including InP quantumdots as the active medium. Using electron beam lithography and a combination of dry- and wet-etch processes, dimers with inter-disk separations down to d < 100 nm have been fabricated. Applying a photo-thermal heating scheme, we overcome the spectral mode detuning due to the size mismatch between the two disks forming the dimer. We observe signatures of mode coupling in the corresponding photoluminescence spectra with coupling energies of up to 0.66 MeV. With the aid of a numerical analysis, we specify the geometrical and physical factors of the microdisk dimer precisely, and reproduce its spectrum with good agreement.

Low temperature electron transport measurements on individual single wall carbon nanotubes are described in this thesis. Carbon nanotubes are small hollow cylinders made entirely out of carbon atoms. At low temperatures (below ~10 K) finite length nanotubes form quantumdots. Because of its small

Low temperature electron transport measurements on individual single wall carbon nanotubes are described in this thesis. Carbon nanotubes are small hollow cylinders made entirely out of carbon atoms. At low temperatures (below ~10 K) finite length nanotubes form quantumdots. Because of its small si

This thesis deals with the investigation of spin relaxation of electrons and holes in small ensembles of self-assembled quantumdots using optical techniques. Furthermore, a method to detect the spin orientation in a single quantumdot was developed in the framework of this thesis. A spin storage device was used to optically generate oriented electron spins in small frequency selected quantumdot ensembles using circularly polarized optical excitation. The spin orientation can be determined by the polarization of the time delayed electroluminescence signal generated by the device after a continuously variable storage time. The degree of spin polarized initialization was found to be limited to 0.6 at high magnetic fields, where anisotropic effects are compensated. The spin relaxation was directly measured as a function of magnetic field, lattice temperature and s-shell transition energy of the quantumdot by varying the spin storage time up to 30 ms. Very long spin lifetimes are obtained with a lower limit of T{sub 1}=20 ms at B=4 T and T=1 K. A strong magnetic field dependence T{sub 1}{proportional_to}B{sup -5} has been observed for low temperatures of T=1 K which weakens as the temperature is increased. In addition, the temperature dependence has been determined with T{sub 1}{proportional_to}T{sup -1}. The characteristic dependencies on magnetic field and temperature lead to the identification of the spin relaxation mechanism, which is governed by spin-orbit coupling and mediated by single phonon scattering. This finding is qualitatively supported by the energy dependent measurements. The investigations were extended to a modified device design that enabled studying the spin relaxation dynamics of heavy holes in self-assembled quantumdots. The measurements show a polarization memory effect for holes with up to 0.1 degree of polarization. Furthermore, investigations of the time dynamics of the hole spin relaxation reveal surprisingly long lifetimes T{sub 1}{sup h

Zinc to cadmium cation exchange of ZnSe quantumdots has been used to produce a series of alloyed Zn1-xCdxSe quantumdots. As x increases and the lowest-energy exciton shifts to the red, the peak initially broadens and then sharpens as x approaches 1. Resonance Raman spectra obtained with excitation near the lowest excitonic absorption peak show a gradual shift of the longitudinal optical phonon peak from 251 cm(-1) in pure ZnSe to 210 cm(-1) in nearly pure CdSe with strong broadening at intermediate compositions. The LO overtone to fundamental intensity ratio, a rough gauge of exciton-phonon coupling strength, increases considerably for intermediate compositions compared with those of either pure ZnSe or pure CdSe. The results indicate that partial localization of the hole in locally Cd-rich regions of the alloyed particles increases the strengths of local internal electric fields, increasing the coupling between the exciton and polar optical phonons.

The problem of tunneling control in systems "quantumdot - quantum well" (as well as "quantumdot - quantumdot" or quantum molecule) and "quantumdot - bulk contact" is studied as a quantum tunneling with dissipation process in the semiclassical (instanton) approximation. For these systems temperature and correlation between a quantumdot radius and a quantum well width (or another quantumdot radius) are considered to be control parameters. The condition for a single electron blockade is fo...

We study the electronic spectrum of coupledquantumdots (QDs) arranged as a graphene hexagonal lattice in the presence of an external perpendicular magnetic Geld. In our tight-binding model, the effect of the magnetic field is included in both the Peierls phase of the Hamiltonian and the tight-binding basis Wannier function. The energy of the system is analyzed when the magnetic flux through the lattice unit cell is a rational fraction of the quantum flux. The calculated spectrum has recursive properties, similar to those of the classical Hofstadter butterfly. However, unlike the ideal Hofstadter butterfly structure, our result is asymmetric since the impacts of the specific material and the magnetic field on the wavefunctions are included, making the results more realistic.%We study the electronic spectrum of coupledquantumdots (QDs) arranged as a graphene hexagonal lattice in the presence of an external perpendicular magnetic field.In our tight-binding model,the effect of the magnetic field is included in both the Peierls phase of the Hamiltonian and the tight-binding basis Wannier function.The energy of the system is analyzed when the magnetic flux through the lattice unit cell is a rational fraction of the quantum flux.The calculated spectrum has recursive properties,similar to those of the classical Hofstadter butterfly.However,unlike the ideal Hofstadter butterfly structure,our result is asymmetric since the impacts of the specific material and the magnetic field on the wavefunctions are included,making the results more realistic.

This Thesis is focused on the study of quantum electrodynamics in photonic crystal waveguides. We investigate the interplay between a single quantumdot and the fundamental mode of the photonic crystal waveguide. We demonstrate experimental coupling eciencies for the spontaneous emission...... into the mode exceeding 98% for emitters spectrally close to the band-edge of the waveguide mode. In addition we illustrate the broadband nature of the underlying eects, by obtaining coupling eciencies above 90% for quantumdots detuned from the band edge by as far as 20nm. These values are in good agreement...... quantum-dot-waveguide coupling. Such a structure is ideally suited for a number of applications in quantum information processing and among others we propose an on-chip spin-photon interface, a single photon transistor, and a deterministic cNOT gate....

This Thesis is focused on the study of quantum electrodynamics in photonic crystal waveguides. We investigate the interplay between a single quantumdot and the fundamental mode of the photonic crystal waveguide. We demonstrate experimental coupling eciencies for the spontaneous emission...... into the mode exceeding 98% for emitters spectrally close to the band-edge of the waveguide mode. In addition we illustrate the broadband nature of the underlying eects, by obtaining coupling eciencies above 90% for quantumdots detuned from the band edge by as far as 20nm. These values are in good agreement...... quantum-dot-waveguide coupling. Such a structure is ideally suited for a number of applications in quantum information processing and among others we propose an on-chip spin-photon interface, a single photon transistor, and a deterministic cNOT gate....

Semiconductor quantumdots are often described as "artificial atoms": They are small nanometre-sized structures in which electrons only are allowed to exist at certain discrete levels due to size quantization, thus allowing the engineering of fundamental properties such as the coupling to light....... The main property of semiconductor quantumdots compared to bulk material or even quantum well structures is the discrete nature of the allowed states, which means that inversion of the medium can be obtained for very low electron densities. This has led to the fabrication of quantumdot lasers with record......-low threshold currents and amplifiers with record-high power levels. In this tutorial we will review the basic properties of quantumdots, emphasizing the properties which are important for laser and amplifier applications, as well as devices for all-optical signal processing. The high-speed properties...

We report InP nanowire photodetectors with a single InAsP quantumdot as light absorbing element. With excitation above the InP band gap, the nanowire photodetectors are efficient (quantum efficiency of 4%). Under resonant excitation of the quantumdot, the photocurrent amplitude depends on the line

We report InP nanowire photodetectors with a single InAsP quantumdot as light absorbing element. With excitation above the InP band gap, the nanowire photodetectors are efficient (quantum efficiency of 4%). Under resonant excitation of the quantumdot, the photocurrent amplitude depends on the

Quantumdots are often called artificial atoms because, like real atoms, they confine electrons to quantized states with discrete energies. However, although real atoms are identical, most quantumdots comprise hundreds or thousands of atoms, with inevitable variations in size and shape and, consequently, unavoidable variability in their wavefunctions and energies. Electrostatic gates can be used to mitigate these variations by adjusting the electron energy levels, but the more ambitious goal of creating quantumdots with intrinsically digital fidelity by eliminating statistical variations in their size, shape and arrangement remains elusive. We used a scanning tunnelling microscope to create quantumdots with identical, deterministic sizes. By using the lattice of a reconstructed semiconductor surface to fix the position of each atom, we controlled the shape and location of the dots with effectively zero error. This allowed us to construct quantumdot molecules whose coupling has no intrinsic variation but could nonetheless be tuned with arbitrary precision over a wide range. Digital fidelity opens the door to quantumdot architectures free of intrinsic broadening-an important goal for technologies from nanophotonics to quantum information processing as well as for fundamental studies of confined electrons.

These sophisticated technologies for the growth of high quality epitaxial layers of compound semiconductor materials on single crystal semiconductor substrates are becoming increasingly important for the development of the semiconductor electronics industry. This article is intended to convey the flavor of the subject by focusing on the technology and applications of self-assembled quantumdots (QDs and to give an introduction to some of the essential characteristics.

This Communication describes the photoredox catalysis of a C-C coupling reaction between 1-phenylpyrrolidine (PhPyr) and phenyl trans-styryl sulfone by visible-light-absorbing colloidal CdS quantumdots (QDs), without a sacrificial oxidant or reductant, and without a co-catalyst. Simple kinetic analysis reveals that photo-oxidation of PhPyr by the QDs is the rate-limiting step. Disordering of the ligand shell of the QDs by creating mixed monolayers of oleate and octylphosphonate increases the initial rate of the reaction by a factor of 2.3, and the energy efficiency (mol product/joule of incident photons) of the reaction by a factor of 1.6, by facilitating the hole-transfer step.

We analyse the transport properties of a coupled double quantumdot (DQD) device with one of the dots (QD1) coupled to metallic leads and the other (QD2) embedded in an Aharonov-Bhom (A-B) ring by means of the slave-boson mean-Geld theory. It is found that in this system, the Kondo resonance and the Fano interference exist simultaneously, the enhancing Kondo effect and the increasing hopping of the QD2-Ring destroy the localized electron state in the QD2 for the QD1-leads, and accordingly, the Fano interference between the DQD-leads and the QD1-leads are suppressed. Under some conditions, the Fano interference can be quenched fully and the single Kondo resonance of the QD1-leads comes into being. Moreover, when the magnetic flux of the A-B ring is zero, the influence of the parity of the A-B ring on the transport properties is very weak, but this inSuence becomes more obvious with non-zero magnetic flux. Thus this model may be a candidate for future device applications.

This book highlights the most recent developments in quantumdot spin physics and the generation of deterministic superior non-classical light states with quantumdots. In particular, it addresses single quantumdot spin manipulation, spin-photon entanglement and the generation of single-photon and entangled photon pair states with nearly ideal properties. The role of semiconductor microcavities, nanophotonic interfaces as well as quantum photonic integrated circuits is emphasized. The latest theoretical and experimental studies of phonon-dressed light matter interaction, single-dot lasing and resonance fluorescence in QD cavity systems are also provided. The book is written by the leading experts in the field.

Quantumdot structures confine electrons in a small region of space. Some properties of semiconductor quantumdots, such as the discrete energy levels and shell filling effects visible in addition spectra, have analogies to those of atoms and indeed dots are sometimes referred to as 'artificial atoms'. However, atoms and dots show some fundamental differences due to electron correlations. For real atoms, the kinetic energy of electrons dominates over their mutual Coulomb repulsion energy and for this reason the independent electron approximation works well. For quantumdots the confining potential may be shallower than that of real atoms leading to lower electron densities and a dominance of mutual Coulomb repulsion over kinetic energy. In this strongly correlated regime the independent electron picture leads to qualitatively incorrect results. This thesis concentrates on few-electron quantumdots in the strongly correlated regime both for quasi-one-dimensional and two-dimensional dots in a square confining p...

Multiexcitonic transitions and emission of several photons per excitation comprise a very attractive feature of semiconductor quantumdots for optoelectronics applications. However, these higher-order radiative processes are usually quenched in colloidal quantumdots by Auger and other nonradiative decay channels. To increase the multiexcitonic quantum efficiency, several groups have explored plasmonic enhancement, so far with moderate results. By controlled positioning of individual quantumdots in the near field of gold nanocone antennas, we enhance the radiative decay rates of monoexcitons and biexcitons by 109 and 100 folds at quantum efficiencies of 60 and 70%, respectively, in very good agreement with the outcome of numerical calculations. We discuss the implications of our work for future fundamental and applied research in nano-optics. PMID:28195140

Multiexcitonic transitions and emission of several photons per excitation comprise a very attractive feature of semiconductor quantumdots for optoelectronics applications. However, these higher-order radiative processes are usually quenched in colloidal quantumdots by Auger and other non-radiative decay channels. To increase the multiexcitonic quantum efficiency, several groups have explored plasmonic enhancement, so far with moderate results. By controlled positioning of individual quantumdots in the near field of gold nanocone antennas, we enhance the radiative decay rates of monoexcitons and biexcitons by 109 and 100 folds at quantum efficiencies of 60% and 70%, respectively, in very good agreement with the outcome of numerical calculations. We discuss the implications of our work for future fundamental and applied research in nano-optics.

Multiexcitonic transitions and emission of several photons per excitation comprise a very attractive feature of semiconductor quantumdots for optoelectronics applications. However, these higher-order radiative processes are usually quenched in colloidal quantumdots by Auger and other nonradiative decay channels. To increase the multiexcitonic quantum efficiency, several groups have explored plasmonic enhancement, so far with moderate results. By controlled positioning of individual quantumdots in the near field of gold nanocone antennas, we enhance the radiative decay rates of monoexcitons and biexcitons by 109 and 100 folds at quantum efficiencies of 60 and 70%, respectively, in very good agreement with the outcome of numerical calculations. We discuss the implications of our work for future fundamental and applied research in nano-optics.

We theoretically investigate controlled tunable all-optical filtering and buffering of optical pulses in a hybrid nano-photonic structure,where a single quantumdot (QD) embedded in a photonic crystal nanocavity is sidecoupled between a bare nanocavity and a photonic crystal waveguide.We demonstrate that there is a sharp low-loss transmission peak in the transmission spectrum under even low QD-nanocavity coupling strength and the input optical pulses can be delayed up to several hundred piceseconds within the dephasing time of the QD.The filtering regime can be shifted readily by manipulating the detuning between the QD excitonic transition frequency and resonant frequency of the nanocavity mode,which can be explored in future for on-clup all-optical logic and signal processing.

We demonstrate a single-photon collection efficiency of (44.3 ± 2.1)% from a quantumdot in a low-Q mode of a photonic-crystal cavity with a single-photon purity of g(2)(0) = (4 ± 5)% recorded above the saturation power. The high efficiency is directly confirmed by detecting up to 962 ± 46...... kilocounts per second on a single-photon detector on another quantumdotcoupled to the cavity mode. The high collection efficiency is found to be broadband, as is explained by detailed numerical simulations. Cavity-enhanced efficient excitation of quantumdots is obtained through phonon-mediated excitation...... and under these conditions, single-photon indistinguishability measurements reveal long coherence times reaching 0.77 ± 0.19 ns in a weak-excitation regime. Our work demonstrates that photonic crystals provide a very promising platform for highly integrated generation of coherent single photons including...

Full text: We investigate the spin polarized transport properties of a nanoscopic device constituted by a quantumdot connected to two leads. The electrical current circulates with a spin polarization that is modulated via a gate potential that controls the intensity of the spin-orbit coupling, the Rashba effect. We study a polarized field-effect transistor when one of its parts is constituted by a small quantumdot, which energies are controlled by another gate potential operating inside the confined region. The high confinement and correlation suffered by the charges inside the dot gives rise to novel phenomena. We show that through the manipulation of the gate potential applied to the dot it is possible to control, in a very efficient way, the intensity and polarization of the current that goes along the system. Other crucial parameters to be varied in order to understand the behavior of this system are the intensity of the external applied electric and magnetic field. The system is represented by the Anderson Impurity Hamiltonian summed to a spin-orbit interaction, which describes the Rashba effect. To obtain the current of this out-of-equilibrium system we use the Keldysh formalism.The solution of the Green function are compatible with the Coulomb blockade regime. We show that under the effect of a external magnetic field, if the dot is small enough the device operates as a complete spin filter that can be controlled by the gate potential. The behavior of this device when it is injected into it a polarized current and modulated by the Rashba effect is as well studied. (author)

Full Text Available Optical nonlinear effects are examined using a two-color micro-photoluminescence (micro-PL method in a coherently coupled exciton-biexciton system in a single quantumdot (QD. PL and photoluminescence excitation spectroscopy (PLE are employed to measure the absorption spectra of the exciton and biexciton states. PLE for Stokes and anti-Stokes PL enables us to clarify the nonlinear optical absorption properties in the lowest exciton and biexciton states. The nonlinear absorption spectra for excitons exhibit asymmetric shapes with peak and dip structures, and provide a distinct contrast to the symmetric dip structures of conventional nonlinear spectra. Theoretical analyses with a density matrix method indicate that the nonlinear spectra are caused not by a simple coherent interaction between the exciton and biexciton states but by coupling effects among exciton, biexciton and continuum states. These results indicate that Fano quantum interference effects appear in exciton-biexciton systems at QDs and offer important insights into their physics.

This paper describes the modeling of quantumdots lasers with the aim of assessing the conditions for stable cw dual-mode operation when the mode separation lies in the THz range. Several possible models suited for InAs quantumdots in InP barriers are analytically evaluated, in particular quantumdots electrically coupled through a direct exchange of excitation by the wetting layer or quantumdots optically coupled through the homogeneous broadening of their optical gain. A stable dual-mode regime is shown possible in all cases when quantumdots are used as active layer whereas a gain medium of quantum well or bulk type inevitably leads to bistable behavior. The choice of a quantumdots gain medium perfectly matched the production of dual-mode lasers devoted to THz generation by photomixing.

The covalent coupling of nanomaterials to bio-recognition molecules is a critical intermediate step in using nanomaterials for biology and medicine. Here we investigate the carbodiimide-mediated conjugation of fluorescent quantumdots to different proteins (e.g., immunoglobulin G, bovine serum albumin, and horseradish peroxidase). To enable these studies, we developed a simple method to isolate quantumdot bioconjugates from unconjugated quantumdots. The results show that the reactant concentrations and protein type will impact the overall number of proteins conjugated onto the surfaces of the quantumdots, homogeneity of the protein-quantumdot conjugate population, quantum efficiency, binding avidity, and enzymatic kinetics. We propose general principles that should be followed for the successful coupling of proteins to quantumdots.

We analyze, from a quantum information theory perspective, the possibility of realizing an SU(4) entangled Kondo regime in semiconductor double quantumdot devices. We focus our analysis on the ground-state properties and consider the general experimental situation where the coupling parameters of the two quantumdots differ. We model each quantumdot with an Anderson-type Hamiltonian including an interdot Coulomb repulsion and tunnel couplings for each quantumdot to independent fermionic baths. We find that the spin and pseudospin entanglements can be made equal, and the SU(4) symmetry recovered, if the gate voltages are chosen in such a way that the average charge occupancies of the two quantumdots are equal, and the double occupancy on the double quantumdot is suppressed. We present density matrix renormalization group numerical results for the spin and pseudospin entanglement entropies, and analytical results for a simplified model that captures the main physics of the problem.

Influence of quantum fluctuations in a system consisting of a quantumdot and the reservoir of acoustic phonons on processes in which the quantumdot takes part is investigated. Under some conditions this influence is shown to be very strong. We find a contribution from the quantum fluctuations to the self-energy function of the exciton coupled to the quantumdot.

Full Text Available Laterally close-packed quantumdots (QDs called quantumdot molecules (QDMs are grown by modified molecular beam epitaxy (MBE. Quantumdots could be aligned and cross hatched. Quantum rings (QRs created from quantumdot transformation during thin or partial capping are used as templates for the formations of bi-quantumdot molecules (Bi-QDMs and quantumdot rings (QDRs.
Preferable quantumdot nanostructure for quantum computation based on quantumdot cellular automata (QCA is laterally close-packed quantumdot molecules having four quantumdots at the corners of square configuration. These four quantumdot sets are called quadra-quantumdots (QQDs. Aligned quadra-quantumdots with two electron confinements work like a wire for digital information transmission by Coulomb repulsion force, which is fast and consumes little power. Combination of quadra-quantumdots in line and their cross-over works as logic gates and memory bits.
Molecular Beam Epitaxial growth technique called 'Droplet Epitaxy' has been developed for several quantum nanostructures such as quantum rings and quantumdot rings. Quantum rings are prepared by using 20 ML In-Ga (15:85 droplets deposited on a GaAs substrate at 390'C with a droplet growth rate of 1ML/s. Arsenic flux (7'8'10-6Torr is then exposed for InGaAs crystallization at 200'C for 5 min. During droplet epitaxy at a high droplet thickness and high temperature, out-diffusion from the centre of droplets occurs under anisotropic strain. This leads to quantum ring structures having non-uniform ring stripes and deep square-shaped nanoholes. Using these peculiar quantum rings as templates, four quantumdots situated at the corners of a square shape are regrown. Two of these four quantumdots are aligned either or, which are preferable crystallographic directions of quantumdot alignment in general.

We exploit the coupled emission-states of a single-chip semiconductor InAs/GaAs quantum-dot laser emitting simultaneously on ground-state (λ(GS) = 1245 nm) and excited-state (λ(ES) = 1175 nm) to demonstrate coupled-two-state self-mixing velocimetry for a moving diffuse reflector. A 13 Hz-narrow Doppler beat frequency signal at 317 Hz is obtained for a reflector velocity of 3 mm/s, which exemplifies a 66-fold improvement in width as compared to single-wavelength self-mixing velocimetry. Simulation results reveal the physical origin of this signal, the coupling of excited-state and ground-state photons via the carriers, which is unique for quantum-dot lasers and reproduce the experimental results with excellent agreement.

Graphical abstract: -- Highlights: •An ultrasensitive ECL immunosensor of CdSe QDs for clenbuterol determination is developed. •The CdSe QDs showed great biocompatibility and could be easier to make direct use of such QDs in the region of biological system. •Enzymatic amplification strategy was proposed by combining the coreactant and pAb/GaRIgG-HRP. •Enzymatic amplification increased ECL emission and extended the analyte in presence of substrate. •It provided a method for detecting clenbuterol and enlarged the usage of QDs in ECL biosensing. -- Abstract: An ultrasensitive electrochemiluminescence (ECL) immunosensor based on CdSe quantumdots (QDs) has been designed for the detection of clenbuterol. The immunosensor was fabricated by layer by layer and characterized with atomic force microscopic images (AFM) and electrochemical impedance spectra (EIS). In oxygen-saturated pH = 9.0 Tris-HCl buffer, a strong ECL emission of QDs could be observed during the cathodic process due to the H{sub 2}O{sub 2} product from electrochemical reduction of dissolved oxygen. Upon the formation of immunocomplex, the second antibody labeled with horseradish peroxidase was simply immobilized on the electrode surface. The ECL emission decreased since steric hindrance of the immunocomplex slowed down the electron-transfer speed of dissolved oxygen, and also could be greatly amplified by an enzymatic cycle to consume the self-produced coreactant. Using clenbuterol as model analyte, the ECL intensity was determined by the concentration of competitive immunoassay of clenbuterol with a wide calibration in the range of 0.05 ng mL{sup −1} to 1000 ng mL{sup −1}, and a low detection limit was 0.02 ng mL{sup −1}. The immunosensor shows good stability and fabrication reproducibility. It was applied to detecting practical samples with the satisfactory results. This immunosensing strategy opens a new avenue for detection of residue and application of QDs in ECL biosensing.

In this paper, we demonstrate theoretically that the Quantumdots are quite interesting for the electronics industry. Semiconductor quantumdots (QDs) are nanometer-scale crystals, which have unique photo physical, quantum electrical properties, size-dependent optical properties, There small size means that electrons do not have to travel as far as with larger particles, thus electronic devices can operate faster. Cheaper than modern commercial solar cells while making use of a wider variety of photon energies, including "waste heat" from the sun's energy. Quantumdots can be used in tandem cells, which are multi junction photovoltaic cells or in the intermediate band setup. PbSe (lead selenide) is commonly used in quantumdot solar cells.

The impact of electromechanical coupling on optical properties of light-emitting diodes (LEDs) with InGaN/GaN quantum-dot (QD) active regions is studied by numerical simulations. The structure, i.e. the shape and the average In content of the QDs, has been directly derived from experimental data on out-of-plane strain distribution obtained from the geometric-phase analysis of a high-resolution transmission electron microscopy image of an LED structure grown by metalorganic vapor-phase epitaxy. Using continuum k\\cdot p calculations, we have studied first the lateral and full electromechanical coupling between the QDs in the active region and its impact on the emission spectrum of a single QD located in the center of the region. Our simulations demonstrate the spectrum to be weakly affected by the coupling despite the strong common strain field induced in the QD active region. Then we analyzed the effect of vertical coupling between vertically stacked QDs as a function of the interdot distance. We have found that QCSE gives rise to a blue-shift of the overall emission spectrum when the interdot distance becomes small enough. Finally, we compared the theoretical spectrum obtained from simulation of the entire active region with an experimental electroluminescence (EL) spectrum. While the theoretical peak emission wavelength of the selected central QD corresponded well to that of the EL spectrum, the width of the latter one was determined by the scatter in the structures of various QDs located in the active region. Good agreement between the simulations and experiment achieved as a whole validates our model based on realistic structure of the QD active region and demonstrates advantages of the applied approach.

the electric 179 180 Microcavity and Photonic Cristal (a) (b) _..•rQ Fig. 1. (a) A schematic diagram of a spherical microcavity with a quantumdot at...core is 2.7 while the refractive indices 182 Microcavity and Photonic Cristal of the layers forming the Bragg reflector are 1.45 and 2.7 and correspond

The present thesis is composed of four different works. All deal with coherent transport through interacting quantumdots, which are tunnel-coupled to external leads. There a two main motivations for the use of quantumdots. First, they are an ideal device to study the influence of strong Coulomb repulsion, and second, their discrete energy levels can easily be tuned by external gate electrodes to create different transport regimes. The expression of coherence includes a very wide range of physical correlations and, therefore, the four works are basically independent of each other. Before motivating and introducing the different works in more detail, we remark that in all works a diagrammatic real-time perturbation theory is used. The fermionic degrees of freedom of the leads are traced out and the elements of the resulting reduced density matrix can be treated explicitly by means of a generalized master equation. How this equation is solved, depends on the details of the problem under consideration. In the first of the four works adiabatic pumping through an Aharonov-Bohm interferometer with a quantumdot embedded in each of the two arms is studied. In adiabatic pumping transport is generated by varying two system parameters periodically in time. We consider the two dot levels to be these two pumping parameters. Since they are located in different arms of the interferometer, pumping is a quantum mechanical effect purely relying on coherent superpositions of the dot states. It is very challenging to identify a quantum pumping mechanism in experiments, because a capacitive coupling of the gate electrodes to the leads may yield an undesired AC bias voltage, which is rectified by a time dependent conductance. Therefore, distinguishing features of these two transport mechanisms are required. We find that the dependence on the magnetic field is the key feature. While the pumped charge is an odd function of the magnetic flux, the rectified current is even, at least in

We demonstrate room temperature visible wavelength photoluminescence from In0.5Ga0.5As quantumdots embedded in a GaP membrane. Time-resolved above band photoluminescence measurements of quantumdot emission show a biexpontential decay with lifetimes of ~200 ps. We fabricate photonic crystal cavities which provide enhanced outcoupling of quantumdot emission, allowing the observation of narrow lines indicative of single quantumdot emission. This materials system is compatible with monolithic integration on Si, and is promising for high efficiency detection of single quantumdot emission as well as optoelectronic devices emitting at visible wavelengths.

@@ Considering the three-dimensional confinement of the electrons and holes and the strong built-in electric field (BEF) in the wurtzite InGaN strained coupledquantumdots (QDs), the positively charged donor bound exciton states and interband optical transitions are investigated theoretically by means of a variational method. Our calculations indicate that the emission wavelengths sensitively depend on the donor position, the strong BEF,and the structure parameters of the QD system.

Using a deterministic single P donor placed with atomic precision accuracy next to a nanoscale silicon quantumdot, we present a way to analyze the energy spectrum of small quantumdots in silicon by tunnel-coupled transport measurements. The energy-level structure of the quantumdot is observed as resonance features within the transport bias triangles when the donor chemical potential is aligned with states within the quantumdot as confirmed by a numeric rate equation solver SIMON. This technique allows us to independently extract the quantumdot level structure irrespective of the density of states in the leads. Such a method is useful for the investigation of silicon quantumdots in the few-electron regime where the level structure is governed by an intricate interplay between the spin- and the valley-orbit degrees of freedom.

Quantumdots are semiconductor nanocrystals that have broad excitation spectra, narrow emission spectra, tunable emission peaks, long fluorescence lifetimes, negligible photobleaching, and ability to be conjugated to proteins, making them excellent probes for bioimaging applications. Here the author reviews the advantages and disadvantages of using quantumdots in bioimaging applications, such as single-particle tracking and fluorescence resonance energy transfer, to study receptor-mediated transport.

The phase transfer of quantumdots to water is an important aspect of preparing nanomaterials that are suitable for biological applications, and although numerous reports describe ligand exchange, very few describe efficient ligand encapsulation techniques. In this report, we not only report a new method of phase transferring quantumdots (QDs) using an amphiphilic protein (hydrophobin) but also describe the advantages of using a biological molecule with available functional groups and their use in imaging cancer cells in vivo and other imaging applications.

Quantumdots are semiconductor nanocrystals that have broad excitation spectra, narrow emission spectra, tunable emission peaks, long fluorescence lifetimes, negligible photobleaching, and ability to be conjugated to proteins, making them excellent probes for bioimaging applications. Here the author reviews the advantages and disadvantages of using quantumdots in bioimaging applications, such as single-particle tracking and fluorescence resonance energy transfer, to study receptor-mediated t...

In this paper we explore the structure parameters, hydrostatic pressure and temperature effects on Nonlinear optical rectification (NOR) in an asymmetric vertically coupled lens-shaped InAs/GaAs quantumdots. During epitaxial growth, lens-shaped quantumdots (QDs) are formed on the wetting layer (WL). Many theoretical works have neglected WL and its effect on nonlinear optical properties of QD-based systems for sake of simplicity. However, in this work the WL has been shown to be so influential in the intersubband energy and nonlinear optical rectification magnitude. Also, a detailed and comprehensive study of the nonlinear optical rectification is theoretical investigated within the framework of the compact density-matrix approach and finite difference method (FDM). It's found that nonlinear optical rectification coefficient is strongly affected not only by the WL, but also by the pressure, temperature and the coupled width between the QDs. Obtained results revealed that a red or a blue shift cane be observed. This behavior in the NOR gives a new degree of freedom in regions of interest for device applications. - Highlights: • Vertically coupled lens-shaped InAs/GaAs quantumdots is investigated. • Photon energy shifts towards the red with increasing pressure. • Photon energy shifts towards the blue with increasing temperature. • Intersubband energy decreases with increasing the wetting layer width. • Nonlinear optical rectification magnitude is controlled and adjusted.

Open quantumdots provide a natural system in which to study both classical and quantum features of transport. From the classical point of view these dots possess a mixed phase space which yields families of closed, regular orbits as well as an expansive sea of chaos. As a closed test bed, they provide a natural system with a very rich set of eigen-states. When coupled to the environment through a pair of quantum point contacts, each of which passes several modes, the original quantum environment evolves into a set of decoherent and coherent states, which eventually couple to the classical states discussed above. The manner of this connection is governed strongly by decoherence theory. The remaining coherent states possess all the properties of pointer states. Here, we discuss the quantum-classical connection and how it appears within the experimental world.

We present a theoretical investigation for the full counting statistics of the Andreev tunneling through a quantumdot (QD) embedded between superconducting (SC) and normal leads in the presence of a strong on-site electron-phonon interaction using nonequilibrium Green function method. For this purpose, we generalize the dressed tunneling approximation (DTA) recently developed in dealing with inelastic tunneling in a normal QD system to the Andreev transport issue. This method takes account of vibrational effect in evaluation of electronic tunneling self energy in comparison with other simple approaches and meanwhile allows us to derive an explicit analytical formula for the cumulant generating function at the subgap region. We then analyze the interplay of polaronic and SC proximity effects on the Andreev reflection spectrum, current-voltage characteristics, and current fluctuations of the hybrid system. Our main findings include: (1) no phonon side peaks in the linear Andreev conductance; (2) a negative differential conductance stemming from the suppressed Andreev reflection spectrum; (3) a novel inelastic resonant peak in the differential conductance due to phonon assisted Andreev reflection; (4) enhancement or suppression of shot noise for the symmetric or asymmetric tunnel-coupling system, respectively.

Full Text Available In this work, the effects of vertical electric field on the electronic and optical properties of strained semi-spheroid-shaped InAs/GaAs quantumdot (QD coupled to its wetting layer (WL aimed to enhance the nonlinear optical properties were investigated. The dependence of energy eigenvalues of S- and P- states and intersubband P-to-S transition energy on applied electric field was studied. A ∼∓ߙ10 meV Stark shift in the intersubband P-to-S transition energy was calculated for a semi-spheroid-shaped QD with height of 5 nm and base-length of 20 nm when bias voltage was varied from 0 V to ±0.8V. The dependence of transition dipole moment and linear and nonlinear optical properties of the system on bias voltage was also studied. It was concluded that increasing the bias voltage from -0.8V to +0.8V leads to increase in figure of merit of the system from ∼0.153 to ∼0.198.

Preparation of a specific quantum state is a required step for a variety of proposed practical uses of quantum dynamics. We report an experimental demonstration of optical quantum state preparation in a semiconductor quantumdot with electrical readout, which contrasts with earlier work based on Rabi flopping in that the method is robust with respect to variation in the optical coupling. We use adiabatic rapid passage, which is capable of inverting single dots to a specified upper level. We d...

A new nanolaser concept using silicon quantumdots (QDs) is proposed.The conduction band opened by the quantum confinement effect gives the pumping levels.Localized states in the gap due to some surface bonds on Si QDs can be formed for the activation of emission.An inversion of population can be generated between the localized states and the valence band in a QD fabricated by using a nanosecond pulse laser.Coupling between the active centres formed by localized states and the defect states of the two-dimensional (2D) photonic crystal can be used to select the model in the nanolaser.

We study coupledquantumdots arranged in a photonic crystal, interacting with light which undergoes a quantum phase transition. At the mean-field level for the infinite lattice, we compute the concurrence of the quantumdots as a measure of their entanglement. We find that this quantity smoothly

量子点（Quantum Dots，QDs）是一种半导体纳米微晶，具有光稳点性好、激发光谱宽而连续、发射光谱窄而对称、荧光寿命长等优点，被广泛应用于生物标记、传感和成像等应用中，水溶性量子点表面带有功能基团，能够与抗体发生偶联，本文着重从共价和非共价偶联两个方面来进行分析，同时对量子点的前景进行展望。%QuantumDots ,a kind of semiconductor nanomaterials,are known as its unique advantages,including sig-nal stability,long fluorescence lifetime. The excitation spectrum is wide and continuous,while the emission spectrum is narrow and symmetric. QDs are widely used in biological labeling,sensing,imaging areas. The hydrosoluble quan-tumdots have many functional groups on surface,and could be conjugated with antibody. We mainly discussed about the methods applied in quantumdots conjugation.

Graphical abstract: The pattern of time evolution of eigenstates of a repulsive impurity doped quantumdot is explored. We have considered Gaussian impurity centers. Under a periodically fluctuating impurity potential, the system reveals a long time dynamics that is undulatory in nature. Coupled to the dopant location, the domain of influence of the impurity potential affects the separation between the eigenstates of the unperturbed system. The investigation points to a threshold value of spatial extension of impurity potential. Above this threshold value, the dopant location becomes important in monitoring the minimum value of impurity potential required to cause excitation. - Abstract: We explore the pattern of time evolution of eigenstates of a repulsive impurity doped quantumdot. The quantumdot is 2-dimensional and contains one electron which is harmonically confined. We have considered Gaussian impurity centers. A static transverse magnetic field is also present. Under a periodically fluctuating impurity potential, the system reveals a long time dynamics that is undulatory in nature. Coupled to the dopant location, the domain of influence of the impurity potential affects the separation between the eigenstates of the unperturbed system. The investigation points to a threshold value of spatial extension of impurity potential. Above this threshold value, the dopant location becomes important in monitoring the minimum value of impurity potential required to cause excitation.

The thermoelectric and the thermospin transport properties,including electrical conductivity,Seebeck coefficient,thermal conductivity,and thermoelectric figure of merit,of a parallel coupled double-quantum-dot Aharonov-Bohm interferometer are investigated by means of the Green function technique.The periodic Anderson model is used to describe the quantumdot system,the Rashba spin-orbit interaction and the Zeeman splitting under a magnetic field are considered.The theoretical results show the constructive contribution of the Rashba effect and the influence of the magnetic field on the thermospin effects.We also show theoretically that material with a high figure of merit can be obtained by tuning the Zeeman splitting energy only.

The main goal of our research is to develop new types of technologically important optically active quantumdot (QD) based materials, study their properties and explore their biological applications. For the first time chiral II-VI QDs have been prepared by us using microwave induced heating with the racemic (Rac), D- and L-enantiomeric forms of penicillamine as stabilisers. Circular dichroism (CD) studies of these QDs have shown that D- and L-penicillamine stabilised particles produced mirror image CD spectra, while the particles prepared with a Rac mixture showed only a weak signal. It was also demonstrated that these QDs show very broad emission bands between 400 and 700 nm due to defects or trap states on the surfaces of the nanocrystals. These QDs have demonstrated highly specific chiral recognition of various biological species including aminoacids. The utilisation of chiral stabilisers also allowed the preparation of new water soluble white emitting CdS nano-tetrapods, which demonstrated circular dichroism in the band-edge region of the spectrum. Biological testing of chiral CdS nanotetrapods displayed a chiral bias for an uptake of the D- penicillamine stabilised nano-tetrapods by cancer cells. It is expected that this research will open new horizons in the chemistry of chiral nanomaterials and their application in nanobiotechnology, medicine and optical chemo- and bio-sensing.

We present an exploration of the spectroscopy of a single quantumdot in a photonic wire. The device presents a high photon extraction efficiency, and strong hybrid coupling to mechanical modes. We use resonance fluorescence to probe the emitter's properties with the highest sensitivity, allowing...

We present results from the spectroscopy of a single quantumdot in a photonic wire. The device presents a high photon extraction efficiency, and strong hybrid coupling to mechanical modes. We use resonance fluorescence to probe the emitter’s properties with the highest sensitivity. Weperform...

A top-gated single-wall carbon nanotube is used to define three coupledquantumdots in series between two electrodes. The additional electron number on each quantumdot is controlled by top-gate voltages allowing for current measurements of single, double, and triple quantumdot stability diagrams....... Simulations using a capacitor model including tunnel coupling between neighboring dots captures the observed behavior with good agreement. Furthermore, anticrossings between indirectly coupled levels and higher order cotunneling are discussed. Udgivelsesdato: April...

We report strongly modified optical emission of InAs/InP quantumdots (QDs) coupled to the surface plasmon resonance (SPR) of In nanoparticles grown by metal-organic vapor phase epitaxy. With increasing In deposition time, the In nanoparticle size increases and the SPR redshifts significantly. When overlapping with the SPR, the excited state photoluminescence of the QDs is strongly enhanced due to QD-SPR coupling while the ground state photoluminescence is quenched due to non-radiative energy transfer. This is underpinned by the wavelength dependence of the spontaneous emission decay time which shows an opposite trend compared to that of bare QDs.

We investigate the light amplification and gain without inversion (GWI) in triple quantumdot molecules in both steady-state and transient state. We demonstrate that the light amplification and GWI of a light pulse can be controlled through the rates of the incoherent pumping and tunneling between electronic levels. The required switching times for switching of a light pulse from absorption to gain and vice versa is then discussed. We obtain switching time at about 40 ps, which resembles a high-speed optical switch in nanostructure. The proposed approach in QDMs may provide some new possibilities for technological applications in optoelectronics and solid-state quantum information science.

We define single quantumdots of lengths varying from 60 nm up to nearly half a micron in Ge-Si core-shell nanowires. The charging energies scale inversely with the quantumdot length between 18 and 4 meV. Subsequently, we split up a long dot into a double quantumdot with a separate control over the tunnel couplings and the electrochemical potential of each dot. Both single and double quantumdot configurations prove to be very stable and show excellent control over the electrostatic environment of the dots, making this system a highly versatile platform for spin-based quantum computing.

Based on the Kubo formula for an electron tunneling junction, we revisit the nonequilibrium transport properties through a quantumdot. Since the Fermi level of the quantumdot is set by the conduction electrons of the leads, we calculate the electron current from the left side by assuming the quantumdotcoupled to the right lead as another side of the tunneling junction, and the other way round is used to calculate the current from the right side. By symmetrizing these two currents, an effective local density states on the dot can be obtained, and is discussed at high and low temperatures, respectively.

deviations. Similar measurements on a quantumdot in a photonic-crystal cavity sow a Rabi splitting on resonance, while time-resolved measurements prove that the system is in the weak coupling regime. Whle tuning the quantumdot through resonance of the high-Q mode we observe a strong and surprisingly...... Successfully model the decay rates with a microscopic model that allows us to for the first time extract the effective phonon density of states, which we can model with bulk phonons. Studies on a quantumdot detuned from a low-Q mode of a photonic-crystal cavity show a high collection efficiency at the first......In this thesis we have performed quantum-electrodynamics experiments on quantumdots embedded in photonic-crystal cavities. We perform a quantitative comparison of the decay dynamics and emission spectra of quantumdots embedded in a micropillar cavity and a photonic-crystal cavity. The light...

Andreev (or Shiba) states in coupled double quantumdots is an open field. Here we demonstrate the realization of Andreev states in double quantumdots in an InSb nanowire coupled to two NbTiN superconductors. The magnetic field dependence of the Andreev states has been explored to resolve the spins in different double dot configurations. The experiment helps to understand the interplay between pair correlation, exchange energy and charging energy with a well-controlled system. It also opens the possibility to implement Majorana modes in Kitaev chains made of such dots.

Chains of quantumdotscoupled to superconductors are promising for the realization of the Kitaev model of a topological superconductor. While individual superconducting quantumdots have been explored, control of longer chains requires understanding of interdot coupling. Here, double quantumdots are defined by gate voltages in indium antimonide nanowires. High transparency superconducting niobium titanium nitride contacts are made to each of the dots in order to induce superconductivity, as well as probe electron transport. Andreev bound states induced on each of dots hybridize to define Andreev molecular states. The evolution of these states is studied as a function of charge parity on the dots, and in magnetic field. The experiments are found in agreement with a numerical model.Quantumdots in a nanowire are one possible approach to creating a solid-state quantum simulator. Here, the authors demonstrate the coupling of electronic states in a double quantumdot to form Andreev molecule states; a potential building block for longer chains suitable for quantum simulation.

We theoretically investigate the influence of radiative coupling on light emission in a photonic crystal slab structure. The calculation method is based on a formalism that combines the photon Green's tensor with a self-consistent Dyson equation approach and is applicable to a wide range of probl......We theoretically investigate the influence of radiative coupling on light emission in a photonic crystal slab structure. The calculation method is based on a formalism that combines the photon Green's tensor with a self-consistent Dyson equation approach and is applicable to a wide range...

Nanoparticle-biomolecules self-assembly is the key to the understanding of biomolecular coating of nanoparticle. However, the self-assembly of biomolecules with nanoparticles is still under-exploited because of the lack of an efficient method to detect the subtle changes in the surface of nanoparticles. In this study, we utilized fluorescence coupled capillary electrophoresis (CE-FL) to probe the binding interaction between a multivalent ligand (dBSA, denatured bovine serum albumin which contains multiple thiol groups) and CdSe/ZnS quantumdots (QDs, 5 nm in diameter). The yield of QDs-dBSA complex increased with increasing molar ratio of dBSA to QDs, which plateaued at a ratio of 8:1. Besides, QDs-dBSA complex showed good stability due to the multivalent interaction, revealing that dBSA is a superior ligand for QDs. The self-assembly kinetics of QDs with dBSA manifested a bi-phasic kinetics with a linear initial stage followed by a saturating stage. This work revealed for the first time that there exist two types of binding sites on the surface of QDs for dBSA: one type termed "high priority" binding sites, which preferentially bind to the protein, whereas the "low priority" sites are occupied only after the first-type binding sites are fully bound. Our work thereby represents the first example of systematic investigation on the details of the metal-affinity driven self-assembly between QDs and dBSA utilizing the high-resolution CE-FL. It also expanded the application of CE-FL in the study of nanoparticle-biomolecule interaction and kinetics analysis.

Stark shifts. This allows us to tune them into mutual resonance with each other and a photonic crystal nanocavity mode as a bias voltage is varied. Photoluminescence measurements show a characteristic triple peak during the double anticrossing, which is a clear signature of a coherently coupled system...

thinned using a thinner to obtain one micron thick films. The dye doped films were later patterned using either EBL or photolithography. Scanning...established for the microdisk resonators. Scanning electron microscope images of single and coupled microring resonators fabricated using EBL are shown

Chiral nanostructures from metals and semiconductors attract wide interest as components for polarization-enabled optoelectronic devices. Similarly to other fields of nanotechnology, graphene-based materials can greatly enrich physical and chemical phenomena associated with optical and electronic properties of chiral nanostructures and facilitate their applications in biology as well as other areas. Here, we report that covalent attachment of l/d-cysteine moieties to the edges of graphene quantumdots (GQDs) leads to their helical buckling due to chiral interactions at the "crowded" edges. Circular dichroism (CD) spectra of the GQDs revealed bands at ca. 210-220 and 250-265 nm that changed their signs for different chirality of the cysteine edge ligands. The high-energy chiroptical peaks at 210-220 nm correspond to the hybridized molecular orbitals involving the chiral center of amino acids and atoms of graphene edges. Diverse experimental and modeling data, including density functional theory calculations of CD spectra with probabilistic distribution of GQD isomers, indicate that the band at 250-265 nm originates from the three-dimensional twisting of the graphene sheet and can be attributed to the chiral excitonic transitions. The positive and negative low-energy CD bands correspond to the left and right helicity of GQDs, respectively. Exposure of liver HepG2 cells to L/D-GQDs reveals their general biocompatibility and a noticeable difference in the toxicity of the stereoisomers. Molecular dynamics simulations demonstrated that d-GQDs have a stronger tendency to accumulate within the cellular membrane than L-GQDs. Emergence of nanoscale chirality in GQDs decorated with biomolecules is expected to be a general stereochemical phenomenon for flexible sheets of nanomaterials.

@@ We investigate the nonlinear thermal transport properties of a single interacting quantumdot with two energy levels tunnel-coupled to two electrodes using nonequilibrium Green function method and Hartree-Fock decoupling approximation. In the case of asymmetric tunnel-couplings to two electrodes, for example, when the upper level of the quantumdot is open for transport, whereas the lower level is blocked, our calculations predict a strong asymmetry for the heat (energy) current, which shows that the quantumdot system may act as a thermal rectifier in this specific situation.

We present a combined experimental and simulation study of a single self-assembled InGaAs quantumdotcoupled to a nearby ($\\sim 25nm$) plasmonic antenna. Micro-photoluminescence spectroscopy shows a $\\sim 2.4\\times$ increase of intensity, which is attributed to spatial far-field redistribution of the emission from the quantumdot-antenna system. Power-dependent studies show similar saturation powers of $2.5\\mu W$ for both coupled and uncoupled quantumdot emission in polarization-resolved me...

The spontaneous emission from self-assembled semiconductor quantumdots is strongly influenced by the environment in which they are placed. This can be used to determine fundamental optical properties of the quantumdots as well as to manipulate and control the quantum-dot emission itself....

The propagation of a coherent optical linear wave in an ensemble of semiconductor quantumdots is considered. It is shown that a distribution of transition dipole moments of the quantumdots changes significantly the polarization and Beer's absorption length of the ensemble of quantumdots. Explicit analytical expressions for these quantities are presented.

We have designed two sets of aldehyde- and azide-modified ligands; these ligands also present lipoic acid anchors and PEG hydrophilic moieties (LA-PEG-CHO and LA-PEG-azide). We combined this design with a photoligation strategy to prepare QDs with good control over the fraction of intact reactive groups per nanocrystal. We first applied the extremely efficient hydrazone coupling ligation to react the QD with hydrozinopyridine, which produces a well-defined absorption feature at 354 nm ascribed to the hydrazone chromophore. We exploited this signature to measure the number of aldehyde groups per QD when the fraction of LA-PEG-CHO per nanocrystal was varied, by comparing the optical signature at 354 with the molar extinction coefficient of the chromophore. This allowed us to extract an estimate for the number of LA-PEG ligand per QDs for a few distinct size nanocrystals. We further complemented these findings with the use of NMR spectroscopy to estimate of the ligand density using well defined signatures of the terminal protons of the ligands, and found a good agreement between the two techniques. We then showed that bio-orthogonal reactions based on CLICK and hydrazone coupling can be achieved using QDs presenting a mixture of azide and CHO functions. We anticipate that this strategy could be applied other nanoparticles such as those of Au and metals and semiconductor nanocrystals.

We develop a theory allowing one to calculate the energy spectra and wave functions of collective excitations in twoand three-dimensional quantum-dot supercrystals. We derive analytical expressions for the energy spectra of twodimensional supercrystals with different Bravias lattices, and use them to analyze the possibility of engineering the supercrystals' band structure. We demonstrate that the variation of the supercrystal's parameters (such as the symmetry of the periodic lattice and the properties of the quantumdots or their environment) enables an unprecedented control over its optical properties, thus paving a way towards the development of new nanophotonics materials.

Highly conjugated multiwalled carbon nanotube-quantumdot heterojunctions were synthesized by ethylene carbodiimide coupling procedure. The functional multiwalled carbon nanotube with carboxylic groups on sidewall could react with the amino group of L-cysteine capped CdSe quantumdots and then resulted in nanotube-quantumdot heterojunctions. Scanning electron microscopy was used to characterize the heterojunctions.

The strong coupling between electronic transport in a single-level quantumdot and a capacitively coupled nanomechanical oscillator may lead to a transition towards a mechanically bistable and blocked-current state. Its observation is at reach in carbon-nanotube state-of-art experiments. In a recent publication [Phys. Rev. Lett. 115, 206802 (2015), 10.1103/PhysRevLett.115.206802] we have shown that this transition is characterized by pronounced signatures on the oscillator mechanical properties: the susceptibility, the displacement fluctuation spectrum, and the ring-down time. These properties are extracted from transport measurements, however the relation between the mechanical quantities and the electronic signal is not always straightforward. Moreover the dependence of the same quantities on temperature, bias or gate voltage, and external dissipation has not been studied. The purpose of this paper is to fill this gap and provide a detailed description of the transition. Specifically we find (i) the relation between the current-noise and the displacement spectrum; (ii) the peculiar behavior of the gate-voltage dependence of these spectra at the transition; (iii) the robustness of the transition towards the effect of external fluctuations and dissipation.

The transport properties of a single plasmon interacting with a hybrid system composed of a semiconductor quantumdot (SQD) and a metal nanoparticle (MNP) coupled to a one-dimensional surface plasmonic waveguide are investigated theoretically via the real-space approach. We considered that the MNP-SQD interaction leads to the formation of a hybrid exciton and the transmission and reflection of a single incident plasmon could be controlled by adjusting the frequency of the classical control field applied to the MNP-SQD hybrid nanosystem, the kinds of MNPs and the background media. The transport properties of a single plasmon interacting with such a hybrid nanosystem discussed here could find applications in the design of next-generation quantum devices, such as single-photon switching and nanomirrors, and in quantum information processing.

The transport properties of a single plasmon interacting with a hybrid system composed of a semiconductor quantumdot (SQD) and a metal nanoparticle (MNP) coupled to a one-dimensional surface plasmonic waveguide are investigated theoretically via the real-space approach. We considered that the MNP-SQD interaction leads to the formation of a hybrid exciton and the transmission and reflection of a single incident plasmon could be controlled by adjusting the frequency of the classical control field applied to the MNP-SQD hybrid nanosystem, the kinds of MNPs and the background media. The transport properties of a single plasmon interacting with such a hybrid nanosystem discussed here could find applications in the design of next-generation quantum devices, such as single-photon switching and nanomirrors, and in quantum information processing.

Semiconductor quantumdots have paid much attention as it is a promising candidate for quantum, optical devices, such as quantum computer and quantumdot laser. We propose a local emission energy control method of semiconductor quantumdots using applying strain by volume expansion of phase change material. Phase change material can change its phase crystalline to amorphous, and the volume expand by its phase change. This method can control energy shift direction and amount by amorphous religion and depth. Using this method, we matched emission energy of two InAs/InP quantumdots. This achievement can connect to observing superradiance phenomenon and quantumdotcoupling effect.

Measured and calculated results are presented on the emission properties of a new class of emitters operating in the cavity quantum electrodynamics regime. The structures are based on high-finesse GaAs/AlAs micropillar cavities, each with an active medium consisting of a layer of InGaAs quantumdots and distinguishing feature of having substantial fraction of spontaneous emission channeled into one cavity mode (high-beta factor). This paper shows that the usual criterion for lasing with a conventional (low-beta factor) cavity, a sharp nonlinearity in an input-output curve accompanied by noticeable linewidth narrowing, has to be reinforced by the equal-time second-order photon autocorrelation function for confirming lasing. It will also show that the equal-time second-order photon autocorrelation function is useful for recognizing superradiance, a manifestation of the correlations possible in high- microcavities operating with quantumdots. In terms of consolidating the collected data and identifying the physi...

We study the fundamental limit on single-photon indistinguishability imposed by decoherence due to phonon interactions in semiconductor quantumdot-cavity quantum electrodynamics systems. Employing an exact diagonalization approach we find large differences compared to standard methods...

Semiconductor quantumdots (QDs) enable efficient coupling between light and matter, which is useful in applications such as light-harvesting and all-solid-state quantum information processing. This coupling can be increased by placing QDs in nanostructured optical environments such as photonic...

The main theme of this thesis is the hyperfine interaction between the many lattice nuclear spins and electron spins localized in GaAs quantumdots. This interaction is an intrinsic property of the material. Despite the fact that this interaction is rather weak, it can, as shown in this thesis, stro

Solar cells based on solution-processed semiconductor nanoparticles -- colloidal quantumdots -- have seen rapid advances in recent years. By offering full-spectrum solar harvesting, these cells are poised to address the urgent need for low-cost, high-efficiency photovoltaics.

QuantumDots (QDs) are semiconductor nanocrystals with distinct photophysical properties finding applications in biology, biosensing, and optoelectronics. Polymeric coatings of QDs are used primarily to provide long-term colloidal stability to QDs dispersed in solutions and also as a source of addit

This final report describes the activities undertaken under grant "Optical Two-Dimensional Spectroscopy of Disordered Semiconductor Quantum Wells and QuantumDots". The goal of this program was to implement optical 2-dimensional Fourier transform spectroscopy and apply it to electronic excitations, including excitons, in semiconductors. Specifically of interest are quantum wells that exhibit disorder due to well width fluctuations and quantumdots. In both cases, 2-D spectroscopy will provide information regarding coupling among excitonic localization sites.

Generation and control of quantum entanglement are studied in an equivalent-neighbor system of spatially-separated semiconductor quantumdotscoupled by a single-mode cavity field. Generation of genuinely multipartite entanglement of qubit states realized by conduction-band electron-spin states in quantumdots is discussed. A protocol for quantum teleportation of electron-spin states via cavity decay is briefly described.

A thermo-responsive polymer/quantumdot platform based on poly(N-isopropyl acrylamide) (PNIPAM) brushes 'grafted from' a gold substrate and quantumdots (QDs) covalently attached to the PNIPAM layer is presented. The PNIPAM brushes are grafted from the gold surface using an iniferter-initiated contr

Colloidal quantumdots (CQDs) have attracted significant interest for applications in electronic and optoelectronic devices such as photodetectors, light emitting diodes, and solar cells. However, a poor understanding of charge transport in these nanocrystalline films hinders their practical applications. The photocarrier radiometry (PCR) technique, a frequency-domain photoluminescence method spectrally gated for radiative recombination photon emissions and exclusion of thermal infrared photons, has been applied to a coupled PbS CQD thin film with inter-dot spacing of 0.5 nm to 1 nm for the analysis of charge transport properties. As the nanoparticle bandgap depends on the size of the quantumdots, polydispersity of the CQD thin film causes bandgap variability leading to photoexcited carrier (exciton) decay lifetime broadening and temperature dependence. The carrier transport mechanisms of QDs are quite different from bulk semiconductors, so the conventional carrier-diffusion wave-based PCR theory was modified into a non-diffusive limit model. A developed variational discrete lifetime reconstruction approach was used to analyze PCR frequency scans under two optical excitation modes: a modulated laser source without, and with, an additional continuous laser source. Using this model, the CQD mean lifetime values were found and variational discrete lifetime spectra were reconstructed.

To investigate the coupled effects of solution chemistry and hydrodynamics on the mobility of quantumdot (QD) nanoparticles in the vadose zone, laboratory scale transport experiments involving single and/or sequential infiltrations of QDs in unsaturated and saturated porous media, and computations of total interaction and capillary potential energies were performed. As ionic strength increased, QD retention in the unsaturated porous media increased; however, this retention was significantly suppressed in the presence of a non-ionic surfactant in the infiltration suspensions as indicated by surfactant enhanced transport of QDs. In the vadose zone, the non-ionic surfactant limited the formation of QD aggregates, enhanced QD mobility and transport, and lowered the solution surface tension, which resulted in a decrease in capillary forces that not only led to a reduction in the removal of QDs, but also impacted the vadose zone flow processes. When chemical transport conditions were favorable (ionic strength of 5 × 10 -4 M and 5 × 10 -3 M, or ionic strengths of 5 × 10 -2 M and 0.5 M with surfactant), the dominating phenomena controlling the mobility and transport of QDs in the vadose zone were meso-scale processes, where infiltration by preferential flow results in the rapid transport of QDs. When chemical transport conditions were unfavorable (ionic strength of 5 × 10 -2 M and 0.5 M) the dominating phenomena controlling the mobility and transport of QDs in the vadose zone were pore-scale processes governed by gas-water interfaces (GWI) that impact the mobility of QDs. The addition of surfactant enhanced the transport of QDs both in favorable and unfavorable chemical transport conditions. The mobility and retention of QDs was controlled by interaction and capillary forces, with the latter being the most influential. GWI were found to be the dominant mechanism and site for QD removal compared with solid-water interfaces (SWI) and pore straining. Additionally

The present thesis is composed of four different works. All deal with coherent transport through interacting quantumdots, which are tunnel-coupled to external leads. There a two main motivations for the use of quantumdots. First, they are an ideal device to study the influence of strong Coulomb repulsion, and second, their discrete energy levels can easily be tuned by external gate electrodes to create different transport regimes. The expression of coherence includes a very wide range of physical correlations and, therefore, the four works are basically independent of each other. Before motivating and introducing the different works in more detail, we remark that in all works a diagrammatic real-time perturbation theory is used. The fermionic degrees of freedom of the leads are traced out and the elements of the resulting reduced density matrix can be treated explicitly by means of a generalized master equation. How this equation is solved, depends on the details of the problem under consideration. In the first of the four works adiabatic pumping through an Aharonov-Bohm interferometer with a quantumdot embedded in each of the two arms is studied. In adiabatic pumping transport is generated by varying two system parameters periodically in time. We consider the two dot levels to be these two pumping parameters. Since they are located in different arms of the interferometer, pumping is a quantum mechanical effect purely relying on coherent superpositions of the dot states. It is very challenging to identify a quantum pumping mechanism in experiments, because a capacitive coupling of the gate electrodes to the leads may yield an undesired AC bias voltage, which is rectified by a time dependent conductance. Therefore, distinguishing features of these two transport mechanisms are required. We find that the dependence on the magnetic field is the key feature. While the pumped charge is an odd function of the magnetic flux, the rectified current is even, at least in

Recently it was proposed to make a quantum bit using the spin of an electron in a quantumdot. We present the first experimental steps towards realizing a system of two coupled qubits. The Zeeman splitting between the two spin states defining the qubit is measured for a one-electron dot in a parallel magnetic field. For a two-electron dot, we control the spin singlet-triplet energy difference with a perpendicular magnetic field, and we induce a transition from singlet to triplet ground state. We find relaxation from triplet to singlet to be extremely slow (> 1 mus), which is promising for quantum computing. We couple two few-electron dots, creating the first fully tunable few-electron double dot. Its charge configuration can be read out with a nearby QPC acting as an integrated charge detector.

In recent years much effort has been devoted to the use of semiconductor quantum dotsystems as building blocks for solid-state-based quantum logic devices. One importantparameter for such devices is the coherence time, which determines the number ofpossible quantum operations. From earlier...... measurements the coherence time of the selfassembledquantum dots (QDs) has been reported to be limited by the spontaneousemission rate at cryogenic temperatures1.In this project we propose to alter the coherence time of QDs by taking advantage of arecent technique on modifying spontaneous emission rates...

In recent years much effort has been devoted to the use of semiconductor quantum dotsystems as building blocks for solid-state-based quantum logic devices. One importantparameter for such devices is the coherence time, which determines the number ofpossible quantum operations. From earlier...... measurements the coherence time of the selfassembledquantum dots (QDs) has been reported to be limited by the spontaneousemission rate at cryogenic temperatures1.In this project we propose to alter the coherence time of QDs by taking advantage of arecent technique on modifying spontaneous emission rates...

As molecular labels for cells and tissues, fluorescent probes have shaped our understanding of biological structures and processes. However, their capacity for quantitative analysis is limited because photon emission rates from multicolour fluorophores are dissimilar, unstable and often unpredictable, which obscures correlations between measured fluorescence and molecular concentration. Here we introduce a new class of light-emitting quantumdots with tunable and equalized fluorescence brightness across a broad range of colours. The key feature is independent tunability of emission wavelength, extinction coefficient and quantum yield through distinct structural domains in the nanocrystal. Precise tuning eliminates a 100-fold red-to-green brightness mismatch of size-tuned quantumdots at the ensemble and single-particle levels, which substantially improves quantitative imaging accuracy in biological tissue. We anticipate that these materials engineering principles will vastly expand the optical engineering landscape of fluorescent probes, facilitate quantitative multicolour imaging in living tissue and improve colour tuning in light-emitting devices.

-dimensional quantumdot crystals. The analyzed SiGe quantumdots have a type II band alignment, with holes confined in the dots and electrons confined in the strained Si in the surrounding of the dots. The recombination energy of these indirect excitons depends on size, Ge content and strain distribution of the quantumdots. It is shown that the structural uniformity of the created quantumdot structures is reflected in their optical properties, resulting in a narrow and stable photoluminescence emission with well separated no-phonon and transversal optical phonon lines. The narrow dot luminescence can be shifted by varying Ge coverage, dot size or dot period. Furthermore excitation-power dependent and temperature dependent photoluminescence measurements are discussed. Band structure calculations indicate that the electronic states of the quantumdot crystals are electronically coupled at least in vertical direction. For the quantumdot crystal with a lateral period of 35 nm even a coupling in all three dimensions is calculated. Thus, the three-dimensional dot arrangement represents not only from the structural but also from the electronic point of view an artificial crystal. (orig.)

Key recent results obtained in studies of a new class of luminophores, colloidal quantumdots, are analyzed. Modern methods for the synthesis and post-synthetic treatment of colloidal quantumdots that make it possible to achieve record high quantum yield of luminescence and to modify their characteristics for specific applications are considered. Currently important avenues of research on colloidal quantumdots and the problems in and prospects for their practical applications in various fields are discussed. The bibliography includes 272 references.

We present a combined experimental and simulation study of a single self-assembled InGaAs quantumdotcoupled to a nearby (˜25 nm) plasmonic antenna. Microphotoluminescence spectroscopy shows a ˜2.4× increase of intensity, which is attributed to spatial far-field redistribution of the emission from the quantumdot-antenna system. Power-dependent studies show similar saturation powers of 2.5 μW for both coupled and uncoupled quantumdot emission in polarization-resolved measurements. Moreover, time-resolved spectroscopy reveals the absence of Purcell enhancement of the quantumdotcoupled to the antenna as compared with an uncoupled dot, yielding comparable exciton lifetimes of τ˜0.5 ns. This observation is supported by numerical simulations, suggesting only minor Purcell-effects of antenna separations >25 nm. The observed increased emission from a coupledquantumdot-plasmonic antenna system is found to be in good qualitative agreement with numerical simulations and will lead to a better understanding of light-matter coupling in such semiconductor-plasmonic hybrid systems.

Full Text Available We describe our recent experimental results on the fabrication of quantum-dot devices in a graphene-based two-dimensional system. Graphene samples were prepared by micromechanical cleavage of graphite crystals on a SiO2/Si substrate. We performed micro-Raman spectroscopy measurements to determine the number of layers of graphene flakes during the device fabrication process. By applying a nanofabrication process to the identified graphene flakes, we prepared a double-quantum-dot device structure comprising two lateral quantumdotscoupled in series. Measurements of low-temperature electrical transport show the device to be a series-coupled double-dot system with varied interdot tunnel coupling, the strength of which changes continuously and non-monotonically as a function of gate voltage.

The ability to control the interaction between nitrogen-vacancy centres in diamond and photonic and/or broadband plasmonic nanostructures is crucial for the development of solid-state quantum devices with optimum performance. However, existing methods typically employ top-down fabrication, which restrict scalable and feasible manipulation of nitrogen-vacancy centres. Here, we develop a general bottom-up approach to fabricate an emerging class of freestanding nanodiamond-based hybrid nanostructures with external functional units of either plasmonic nanoparticles or excitonic quantumdots. Precise control of the structural parameters (including size, composition, coverage and spacing of the external functional units) is achieved, representing a pre-requisite for exploring the underlying physics. Fine tuning of the emission characteristics through structural regulation is demonstrated by performing single-particle optical studies. This study opens a rich toolbox to tailor properties of quantum emitters, which can facilitate design guidelines for devices based on nitrogen-vacancy centres that use these freestanding hybrid nanostructures as building blocks.

exchange-correlation interactions among the confined carriers inside the dots are suggested to be responsible. A density functional calculation for BGR of the ground state transition shows good agreement with our experimental results, especially in the high dot occupancy regime. Many-particle state scattering gives rise to large homogeneous spectral broadening of the PL peaks, from which an intradot relaxation time approx 300 fs is estimated. This observation supports the results obtained by direct excitation of carriers within the QDs. Femtosecond time-resolved photoluminescence measured by frequency up-conversion has been used to investigate carrier dynamics in InAs/GaAs self-assembled quantumdots (QDs). Our results reveal ultrafast carrier relaxation and sequential state filling. Carrier relaxation is proposed to occur by Auger-type processes, and the sequential state filling suggests that intradot relaxation is much faster than carrier capture from the InAs wetting layer. Measurements obtained by direct ...

Quantumdots in GaAs/InGaAs structures have been proposed as a candidate system for realizing quantum computing. The short coherence time of the electronic quantum state that arises from coupling to the nuclei of the substrate is dramatically increased if the system is subjected to a magnetic field and to repeated optical pulsing. This enhancement is due to mode locking: oscillation frequencies resonant with the pulsing frequencies are enhanced, while off-resonant oscillations eventually die out. Because the resonant frequencies are determined by the pulsing frequency only, the system becomes immune to frequency shifts caused by the nuclear coupling and by slight variations between individual quantumdots. The effects remain even after the optical pulsing is terminated. In this work, we explore the phenomenon of mode locking from a quantum mechanical perspective. We treat the dynamics using the central-spin model, which includes coupling to 10-20 nuclei and incoherent decay of the excited electronic state, in a perturbative framework. Using scaling arguments, we extrapolate our results to realistic system parameters. We estimate that the synchronization to the pulsing frequency needs time scales in the order of 1 s .

Light absorption and electron injection are important criteria determining solar energy conversion efficiency. In this research, monodisperse CdSe quantumdots (QDs) are synthesized with five different diameters, and the size-dependent solar energy conversion efficiency of CdSe quantumdot sensitized solar cell (QDSSCs) is investigated by employing the atomic inorganic ligand, S(2-). Absorbance measurements and transmission electron microscopy show that the diameters of the uniform CdSe QDs are 2.5, 3.2, 4.2, 6.4, and 7.8 nm. Larger CdSe QDs generate a larger amount of charge under the irradiation of long wavelength photons, as verified by the absorbance results and the measurements of the external quantum efficiencies. However, the smaller QDs exhibit faster electron injection kinetics from CdSe QDs to TiO2 because of the high energy level of CBCdSe, as verified by time-resolved photoluminescence and internal quantum efficiency results. Importantly, the S(2-) ligand significantly enhances the electronic coupling between the CdSe QDs and TiO2, yielding an enhancement of the charge transfer rate at the interfacial region. As a result, the S(2-) ligand helps improve the new size-dependent solar energy conversion efficiency, showing best performance with 4.2-nm CdSe QDs, whereas conventional ligand, mercaptopropionic acid, does not show any differences in efficiency according to the size of the CdSe QDs. The findings reported herein suggest that the atomic inorganic ligand reinforces the influence of quantum confinement on the solar energy conversion efficiency of QDSSCs.

An amperometric biosensor for highly selective and sensitive determination of methyl parathion (MP) was developed based on dual signal amplification: (1) a large amount of introduced enzyme on the electrode surface and (2) synergistic effects of nanoparticles towards enzymatic catalysis. The fabrication process includes (1) electrochemical deposition of gold nanoparticles by a multi-potential step technique at multiwalled carbon nanotube (MWCNT) film pre-cast on a glassy carbon electrode and (2) immobilization of methyl parathion degrading enzyme (MPDE) onto a modified electrode through CdTe quantumdots (CdTe QDs) covalent attachment. The introduced MWCNT and gold nanoparticles significantly increased the surface area and exhibited synergistic effects towards enzymatic catalysis. CdTe QDs are further used as carriers to load a large amount of enzyme. As a result of these two important enhancement factors, the proposed biosensor exhibited extremely sensitive, perfectly selective, and rapid response to methyl parathion in the absence of a mediator.

We report the fabrication of single and double hole quantumdots using a double-layer-gate design on an undoped accumulation mode {{Al}}x{{Ga}}1-x{As}/GaAs heterostructure. Electrical transport measurements of a single quantumdot show varying addition energies and clear excited states. In addition, the two-level-gate architecture can also be configured into a double quantumdot with tunable inter-dotcoupling.

We have produced GaAs-based quantum-dot edge-emitting lasers operating at 1.16 mu m with record-low transparency current, high output power, and high internal quantum efficiencies. We have also realized GaAs-based quantum-dot lasers emitting at 1.3 mu m, both high-power edge emitters and low...

We report on our recent progress in applying semiconductor quantumdots for spin-based quantum computation, as proposed by Loss and DiVincenzo (1998 Phys. Rev. A 57 120). For the purpose of single-electron spin resonance, we study different types of single quantumdot devices that are designed for

We present a combined experimental and simulation study of a single self-assembled InGaAs quantumdotcoupled to a nearby ($\\sim 25nm$) plasmonic antenna. Micro-photoluminescence spectroscopy shows a $\\sim 2.4\\times$ increase of intensity, which is attributed to spatial far-field redistribution of the emission from the quantumdot-antenna system. Power-dependent studies show similar saturation powers of $2.5\\mu W$ for both coupled and uncoupled quantumdot emission in polarization-resolved measurements. Moreover, time-resolved spectroscopy reveals the absence of Purcell-enhancement of the quantumdotcoupled to the antenna as compared to an uncoupled dot, yielding comparable exciton lifetimes of $\\tau\\sim0.5ns$. This observation is supported by numerical simulations, suggesting only minor Purcell-effects of $25nm$. The observed increased emission from a coupledquantumdot-plasmonic antenna system is found to be in good qualitative agreement with numerical simulations and will lead to a better understanding o...

We performed numerical studies for the conductance of a heterojunction carbon nanotube quantumdot (QD) with an extra spin orbital quantum number and a conventional QD in which the electron state is determined only by the spin quantum number. Our computational approach took into account the spin-orbit interaction and the Coulomb repulsion both between electrons on a QD as well as between the QD electron and the contacts. We utilized an approach based on the Keldysh non-equilibrium Green's function formalism as well as the equation of motion technique. We focused on the case of a finite Coulombic on-site repulsion and considered two possible cases of applied voltage: spin bias and conventional bias. For the system of interest we obtained bias spectroscopy diagrams, i.e. contour charts showing dependence of conductivity on two variables - voltage and the energy level position in a QD - which can be controlled by the plunger gate voltage. The finite Coulombic repulsion splits the density of states into two distinct maxima with the energy separation between them controlled by that parameter. It was also shown that an increase of either the value of the on-site Coulomb repulsion in a QD or the parameter of the Coulomb repulsion between the electrons in the QD and the contacts leads to an overall shift of the density of electronic states dependence toward higher energy values. Presence of the QD-lead interaction yields formation of a new pair of peaks in the differential conductance dependence. We also show that existence of four quantum states in a QD leads to abrupt changes in the density of states. These results could be beneficial for potential applications in nanotube-based amperometric sensors.

We performed numerical studies for the conductance of a heterojunction carbon nanotube quantumdot (QD) with an extra spin orbital quantum number and a conventional QD in which the electron state is determined only by the spin quantum number. Our computational approach took into account the spin-orbit interaction and the Coulomb repulsion both between electrons on a QD as well as between the QD electron and the contacts. We utilized an approach based on the Keldysh non-equilibrium Green's function formalism as well as the equation of motion technique. We focused on the case of a finite Coulombic on-site repulsion and considered two possible cases of applied voltage: spin bias and conventional bias. For the system of interest we obtained bias spectroscopy diagrams, i.e. contour charts showing dependence of conductivity on two variables - voltage and the energy level position in a QD - which can be controlled by the plunger gate voltage. The finite Coulombic repulsion splits the density of states into two distinct maxima with the energy separation between them controlled by that parameter. It was also shown that an increase of either the value of the on-site Coulomb repulsion in a QD or the parameter of the Coulomb repulsion between the electrons in the QD and the contacts leads to an overall shift of the density of electronic states dependence toward higher energy values. Presence of the QD-lead interaction yields formation of a new pair of peaks in the differential conductance dependence. We also show that existence of four quantum states in a QD leads to abrupt changes in the density of states. These results could be beneficial for potential applications in nanotube-based amperometric sensors.

Ultrasmall colloidal lead sulfide quantumdots can increase the open circuit voltages of quantum-dot-based solar cells because of their large energy gap. Their small size and visible or near infrared light-emitting property make them attractive to the applications of biological fluorescence labeling. Through a modified organometallic route, we can synthesize lead sulfide quantumdots as small as 1.6 nm in diameter. The low reaction temperature and the addition of a chloroalkane cosolvent decrease the reaction rate, making it possible to obtain the ultrasmall quantumdots. - Highlights: • Ultrasmall colloidal PbS quantumdots as small as 1.6 nm in diameter are synthesized. • The quantumdots emit red light with photoluminescence peak at 760 nm. • The growth temperature is as low as 50 °C. • Addition of cosolvent 1,2-dichloroethane in the reaction decreases the reaction rate.

We have studied the growth and characteristics of self-organized InGaAs/GaAs quantumdot lasers and their monolithic integration with waveguides and quantum well electroabsorption modulators on Si. Utilizing multiple layers of InAs quantumdots as effective dislocation filters near the GaAs-Si interface, we have demonstrated high performance quantumdot lasers grown directly on Si that exhibit, for the first time, relatively low threshold current (J th = 900 A/cm2), large characteristic temperature (T 0 = 278 K), and output slope efficiency ( >=0.3 W/A). Focused-ion-beam milling has been used to form high-quality facets for the cavity mirror and coupling groove of an integrated laser/waveguide system on Si. We have also achieved a groove-coupled laser/modulator system on Si that exhibits a coupling coefficient greater than 20% and a modulation depth of ~ 100% at 5 V reverse bias.

@@ Electronic tunnelling through a one-dimensional quantumdot chain is theoretically studied, when two leads couple to the individual component quantumdots of the chain arbitrarily. If there are some dangling quantumdots in the chain outside the leads, the electron tunnelling through the quantumdot chain is wholly forbidden while the energy of the incident electron is just equal to the molecular energy levels of the dangling quantumdots,which is known as the antiresonance effect. In addition, the influence of electron interaction on the antiresonance effect is discussed within the Hartree-Fock approximation.

We investigate quantumdots with Rashba spin-orbit coupling in the strongly-correlated regime. We show that the presence of the Rashba interaction enhances the Wigner localization in these systems, making it achievable for higher densities than those at which it is observed in Rashba-free quantum...

The emission of silicon quantumdots is weak when their surface is passivated well. Oxygen or nitrogen on the surface of silicon quantumdots can break the passivation to form localized electronic states in the band gap to generate active centers where stronger emission occurs.From this point of view,we can build up radiative matter for emission.Emissions of various wavelengths can be obtained by controlling the surface bonds of silicon quantumdots.Our experimental results demonstrate that annealing is important in the treatment of the activation,and stimulated emissions at about 600 and 700 nm take place on active silicon quantumdots.

Semiconductor nanocrystals, or quantumdots (QDs), are candidates for biological sensing, photovoltaics, and catalysis due to their unique photophysical properties. The most studied QDs are composed of heavy metals like cadmium and lead. However, this engenders concerns over heavy metal toxicity. To address this issue, numerous studies have explored the development of nontoxic (or more accurately less toxic) quantumdots. In this Review, we select three major classes of nontoxic quantumdots composed of carbon, silicon and Group I-III-VI elements and discuss the myriad of synthetic strategies and surface modification methods to synthesize quantumdots composed of these material systems.

Short-wavelength LEDs, a hot research topic in modern optoelectronics, have attracted tremendous attention in recent years because of their great application potential in both civil and military domains. Compared to conventional metallic surface-plasmons (SPs), carbon nanodot (CD) SPs with less optical loss and low cost, broader SP resonant frequency and good biocompatibility are expected to provide more prominent luminescence enhancement for light emitters. Herein, SP-enhanced near-UV emission quantumdot LEDs (Q-LED) were fabricated via introducing CDs into p-GaN/Al2O3/ZnO Q-LEDs by optimizing the molar ratio of ZnO quantumdots to CDs and a significant enhancement (∼20-fold) of the near-UV electroluminescence (EL) intensity from the ZnO-based Q-LEDs was achieved. Time-resolved spectroscopy studies reveal that the observed luminescence enhancement arises due to the resonant coupling between ZnO excitons and CD SPs. The current study not only demonstrates a feasible way to acquire near-UV emission from all-inorganic Q-LEDs, but also provides an effective strategy to enhance the EL intensity of these QD light emitters, which can further be extended to other types of light-emitting devices to improve EL efficiency.

We propose a rapid ground-state optomechanical cooling scheme in a hybrid system, where a two-level quantumdot (QD) is placed in a single-mode cavity and a nanomechanical resonator (NMR) is also coupled to the cavity via radiation pressure. The cavity is driven by a weak laser field while the QD is driven by another weak laser field. Due to the quantum destructive interference arisen from different transition channels induced by simultaneously driving the QD-cavity system in terms of the two different lasers, two-photon absorption for the cavity field can be effectively eliminated by performing an optimal quantum interference condition. Furthermore, it is demonstrated that the QD-cavity system can be unbalancedly prepared in two single-polariton states with different eigenenergies. If the frequency of the NMR is tuned to be resonant with transition between two single-polariton states, it is found that a fast ground-state cooling for the NMR can also be achieved, even when the QD-cavity system is originally in the moderate-coupling regime. Thus the present ground-state cooling scheme for the NMR may be realized with currently available experimental technology.

This paper examines the thermoelectric response of a dissipative quantum-dot heat engine based on the Anderson-Holstein model in two relevant operating limits, (i) when the dot phonon modes are out of equilibrium, and (ii) when the dot phonon modes are strongly coupled to a heat bath. In the first case, a detailed analysis of the physics related to the interplay between the quantum-dot level quantization, the on-site Coulomb interaction, and the electron-phonon coupling on the thermoelectric performance reveals that an n -type heat engine performs better than a p -type heat engine. In the second case, with the aid of the dot temperature estimated by incorporating a thermometer bath, it is shown that the dot temperature deviates from the bath temperature as electron-phonon interaction in the dot becomes stronger. Consequently, it is demonstrated that the dot temperature controls the direction of phonon heat currents, thereby influencing the thermoelectric performance. Finally, the conditions on the maximum efficiency with varying phonon couplings between the dot and all the other macroscopic bodies are analyzed in order to reveal the nature of the optimum junction.

We will review recent studies performed on InAs quantumdots embedded in GaAs photonic wires, which highlight the strong interest of the photonic wire geometry for quantum optics experiments and quantum optoelectronic devices.......We will review recent studies performed on InAs quantumdots embedded in GaAs photonic wires, which highlight the strong interest of the photonic wire geometry for quantum optics experiments and quantum optoelectronic devices....

Novel optoelectronic systems based on ensembles of semiconductor nanocrystals are addressed in this paper. Colloidal semiconductor quantumdots and related quantum-wire structures have been characterized optically; these optical measurements include those made on self-assembled monolayers of DNA molecules terminated on one end with a common substrate and on the other end with TiO II quantumdots. The electronic properties of these structures are modeled and compared with experiment. The characterization and application of ensembles of colloidal quantumdots with molecular interconnects are considered. The chemically-directed assembly of ensembles of colloidal quantumdots with biomolecular interconnects is demonstrated with quantumdot densities in excess of 10 +17 cm -3. A number of novel photodetectors have been designed based on the combined use of double-barrier quantum-well injectors, colloidal quantumdots, and conductive polymers. Optoelectronic devices including photodetectors and solar cells based on threedimensional ensembles of quantumdots are considered along with underlying phenomena such as miniband formation and the robustness of minibands to displacements of quantumdots in the ensemble.

Recently, the use of stereospecific chiral stabilising molecules has also opened another avenue of interest in the area of quantumdot (QD) research. The main goal of our research is to develop new types of technologically important quantumdot materials containing chiral defects, study their properties and explore their applications. The utilisation of chiral penicillamine stabilisers allowed the preparation of new water soluble white emitting CdS quantum nanostructures which demonstrated circular dichroism in the band-edge region of the spectrum. It was also demonstrated that all three types of QDs (D-, L-, and Rac penicillamine stabilised) show very broad emission bands between 400 and 700 nm due to defects or trap states on the surfaces of the nanocrystals. In this work the chiral CdS based quantum nanostructures have also been doped by copper metal ions and new chiral penicilamine stabilized CuS nanoparticles have been prepared and investigated. It was found that copper doping had a strong effect at low levels in the synthesis of chiral CdS nanostructures. We expect that this research will open new horizons in the chemistry of chiral nanomaterials and their application in biotechnology, sensing and asymmetric synthesis.

In this work, a double quantumdot system is studied whose two dots are electrically insulated from one another and contacted independently with two leads. The geometry is optimized to maximize the capacitive interaction between the dots. The samples are characterized by electrical transport measurements in a dilution refrigerator. It is then studied at different tunnel couplings how the capacitive interaction influences the electrical transport in equilibrium. Under certain conditions correlated tunnel processes can be observed. A simple model is derived that serves to understand these processes. The double quantumdot system is defined in lateral arrangement by reactive ion etching of a two-dimensional electron system located only 50 nm below the surface of a GaAs-AlGaAs heterostructure. The samples are characterized in a dilution refrigerator at 25 mK near the common pinch-off point of all four tunnel barriers. A measurement of the differential equilibrium conductances of both quantumdots as a function of two gate voltages yields a honeycomb-like charge stability diagram. The most important sample characteristic is the ratio between the interaction capacitance and the total capacitance of a single quantumdot. For the optimized sample, this ratio turns out to be larger than one third near the common pinch-off point, with a single-dot charging energy of up to 800 {mu}eV. At more positive gate voltages, the capacitances between the quantumdots and their leads increase more and more, thereby diminishing the charging energy. It is shown for the optimized sample that all capacitance coefficients except the dot-lead capacitances are constant to within considerable accuracy over several Coulomb blockade oscillations. In order to measure correlated electrical transport in equilibrium, special parameter regions are examined in which the charges of both quantumdots cannot fluctuate independently of each other. An analytical formula is derived that describes the

We show that the design of a quantum point contact adjacent to a quantumdot can be optimized to produce maximum sensitivity to single-electron charging in the quantumdot. Our analysis is based on the self-consistent solution of coupled three-dimensional Kohn-Sham and Poisson equations for the

The role of nuclear spins for quantum information processing in quantumdots is theoretically investigated in this thesis. Building on the established fact that the most strongly coupled environment for the potential electron spin quantum bit are the surrounding lattice nuclear spins interacting via the hyperfine interaction, we turn this vice into a virtue by designing schemes for harnessing this strong coupling. In this perspective, the ensemble of nuclear spins can be considered an asset, suitable for an active role in quantum information processing due to its intrinsic long coherence times. We present experimentally feasible protocols for the polarization, i.e. initialization, of the nuclear spins and a quantitative solution to our derived master equation. The polarization limiting destructive interference effects, caused by the collective nature of the nuclear coupling to the electron spin, are studied in detail. Efficient ways of mitigating these constraints are presented, demonstrating that highly polarized nuclear ensembles in quantumdots are feasible. At high, but not perfect, polarization of the nuclei the evolution of an electron spin in contact with the spin bath can be efficiently studied by means of a truncation of the Hilbert space. It is shown that the electron spin can function as a mediator of universal quantum gates for collective nuclear spin qubits, yielding a promising architecture for quantum information processing. Furthermore, we show that at high polarization the hyperfine interaction of electron and nuclear spins resembles the celebrated Jaynes-Cummings model of quantum optics. This result opens the door for transfer of knowledge from the mature field of quantum computation with atoms and photons. Additionally, tailored specifically for the quantumdot environment, we propose a novel scheme for the generation of highly squeezed collective nuclear states. Finally we demonstrate that even an unprepared completely mixed nuclear spin

The effect of Rashba spin-orbit interaction on the electronic, thermodynamic, magnetic and transport properties of a one-electron Gaussian quantumdot is investigated in the presence of a magnetic field and its interaction with the electron spin using the canonical ensemble approach. The temperature-dependent energy, magnetization, susceptibility, specific heat and the persistent current are calculated as a function of the spin-orbit coupling parameter. The results are applied to GaAs, InAs and InSb quantumdots.

The effect of Rashba spin–orbit interaction on the electronic, thermodynamic, magnetic and transport properties of a one-electron Gaussian quantumdot is investigated in the presence of a magnetic field and its interaction with the electron spin using the canonical ensemble approach. The temperature-dependent energy, magnetization, susceptibility, specific heat and the persistent current are calculated as a function of the spin–orbit coupling parameter. The results are applied to GaAs, InAs and InSb quantumdots.

We study the quantum-size effect and the influence of the external magnetic field on the exciton ground state energy in the type-II InP quantum disk, lens and pyramid deposited on a wetting layer and embedded in a GaInP matrix. We show that the charge distribution over and below quantumdot and wetting layer induced by trapped exciton strongly depends on the quantumdot morphology and the strength of the magnetic field.

We investigated the design, fabrication and optical investigation of electrically tunable single quantumdot-photonic crystal defect nanocavities operating in both the weak and strong coupling regimes of the light matter interaction. We demonstrate that the quantum confined Stark effect can be employed to quickly and reversibly switch the dot-cavity coupling, simply by varying a gate voltage. Our results show that exciton transitions from individual dots can be tuned by up to {proportional_to}4 meV relative to the nanocavity mode, before the emission quenches due to carrier tunneling escape from the dots. We directly probe spontaneous emission, irreversible polariton decay and the statistics of the emitted photons from a single-dot nanocavity in the weak and strong coupling regimes. New information is obtained on the nature of the dot-cavity coupling in the weak coupling regime and electrical control of zero dimensional polaritons is demonstrated for the first time. The structures investigated are p-i-n photodiodes consisting of an 180nm thick free-standing GaAs membrane into which a two dimensional photonic crystal is formed by etching a triangular lattice of air holes. Low mode volume nanocavities (V{sub mode}<1.6 ({lambda}/n){sup 3}) are realized by omitting 3 holes in a line to form L3 cavities and a single layer of InGaAs self-assembled quantumdots is embedded into the midpoint of the membrane. The nanocavities are electrically contacted via 35 nm thick p- and n-doped contact layers in the GaAs membrane. In the weak coupling regime, time resolved spectroscopy reveals a {proportional_to}7 x shortening of the spontaneous emission lifetime as the dot is tuned through the nanocavity mode, due to the Purcell effect. Upon strongly detuning the same quantumdot transition from the nanocavity mode we observe an additional {proportional_to}8 x lengthening of the spontaneous emission lifetime. These observations unequivocally highlight two regimes of dot

As a vast number of novel materials in particular inorganic nanoparticles have been invented and introduced to all aspects of life, public concerns about how they might affect our ecosystem and human life continue to arise. Such incertitude roots at a fundamental question of how inorganic nanoparticles self-assemble with biomolecules in solution. Various techniques have been developed to probe the interaction between particles and biomolecules, but very few if any can provide advantages of both rapid and convenient. Herein, we report a systematic investigation on quantumdots (QDs) and protein self-assembly inside a capillary. QDs and protein were injected to a capillary one after another. They were mixed inside the capillary when a high voltage was applied. Online separation and detection were then achieved. This new method can also be used to study the self-assembly kinetics of QDs and protein using the Hill equation, the KD value for the self-assembly of QDs and protein was calculated to be 8.8 μM. The obtained results were compared with the previous out of-capillary method and confirmed the effectiveness of the present method.

Here we report a highly efficient novel photocatalyst consisting of Cu2O quantumdots (QDs) incorporated into three-dimensional (3D) flower-like hierarchical BiOBr (hereafter designated QDs-Cu2O/BiOBr), which were synthesized via a simple reductive solution chemistry route and applied to decontaminate the hazardous wastewater containing phenol and organic dyes. The deposition of Cu2O QDs onto the surface of the BiOBr was confirmed by structure and composition characterizations. The QDs-Cu2O/BiOBr composites exhibited superior activity for organic contaminant degradation under visible light and 3 wt% QDs-Cu2O/BiOBr composite showed the highest degrade rate for phenol and methylene blue (MB), which was 11.8 times and 1.4 times than that of pure BiOBr, indicated the QDs-Cu2O/BiOBr composite has the great potential application in purifying hazardous organic contaminant. The incorporated Cu2O QDs played an important role in improving the photocatalytic performance, due to the enhancement of visible light absorption efficiency as well as the efficient separation of the photogenerated charge carriers originating from the intimately contacted interface and the well-aligned band-structures, which was confirmed by the results of PL, photocurrent and EIS measurements. The possible photocatalytic mechanism was proposed based on the experiments and theoretical results.

We investigate the conductance spectra of coupledquantumdots to study systematically the nuclear spin relaxation of different geometries of a two-dimensional network of quantumdots and observe spin blockade dependence on the electronic configurations. We derive the conductance using the Beenakker approach generalized to an array of quantumdots where we consider the nuclear spin transfer to electrons by hyperfine coupling. This allows us to predict the relevant memory effects on the different electronic states by studying the evolution of the single electron resonances in the presence of nuclear spin relaxation. We find that the gradual depolarization of the nuclear system is imprinted in the conductance spectra of the multidot system. Our calculations of the temporal evolution of the conductance resonance reveal that spin blockade can be lifted by hyperfine coupling.

Simulation of quantumdots (QD) is usually performed on the basis of abrupt changes between neighboring materials. In practice, it is not possible to construct such QD because in a real structure the interface between two adjacent materials is not a step. In the work discussed in this paper, vertically coupled InAs/GaAs quantumdots (VCQD) with a non-abrupt change between two neighboring materials are considered. A potential function in the form of a Gaussian distribution was used to show this effect. We also focused on studying the effect of structure, applied electric ( F) and magnetic ( B) fields, pressure ( P), and temperature ( T) on second-harmonic generation (SHG). The analytical expression for SHG was investigated theoretically by use of the density matrix approach, the effective mass, and the finite-difference method (FDM). It was found that the major resonant peak value of SHG is a non-monotonic function of the barrier width ( L B). Moreover, the major resonant peak of SHG is blue-shifted (red-shifted) and its magnitude increases (decreases) monotonically with increasing temperature (pressure). The results obtained also show that the magnitude and position of the resonant peaks of SHG are affected by changes in external conditions, for example applied electric and magnetic fields, structural dimensions of the coupled QD system, and relaxation time ( T 0). Calculations also show that SHG in a VCQD structure with a non-abrupt potential change can be controlled and optimized by appropriate choice of structural dimensions and the external conditions mentioned above.

Two-qubit interactions are at the heart of quantum information processing. For single-spin qubits in semiconductor quantumdots, the exchange gate has always been considered the natural two-qubit gate. The recent integration of a magnetic field or g-factor gradients in coupledquantumdot systems

This thesis describes the synthesis and spectroscopy of CdSe and CdTe semiconductor quantumdots (QDs). The first chapter gives an introduction into the unique size dependent properties of semiconductor quantumdots. Highly luminescent QDs of CdSe and CdTe were prepared via a high temperature method

0K. in a second phase of this contract we turned our efforts to the fabrication and studies of self assembled quantumdots . We first demonstrated a...method for producing InAs-GasAs self assembled quantumdots (SAD) using MBE. (AN)

Colloidal nanocrystal quantumdots comprising an inner core having an average diameter of at least 1.5 nm and an outer shell, where said outer shell comprises multiple monolayers, wherein at least 30% of the quantumdots have an on-time fraction of 0.80 or greater under continuous excitation conditions for a period of time of at least 10 minutes.

This thesis presents different optical experiments performed on semiconductor quantumdots. These structures allow to confine a small number of electrons and holes to a tiny region of space, some nm across. The aim of this work was to study the basic properties of different types of quantumdots

This thesis presents different optical experiments performed on semiconductor quantumdots. These structures allow to confine a small number of electrons and holes to a tiny region of space, some nm across. The aim of this work was to study the basic properties of different types of quantumdots mad

The distribution of waiting times between successive tunneling events is an already established method to characterize current fluctuations in mesoscopic systems. Here, I investigate mechanisms generating correlations between subsequent waiting times in two model systems, a pair of capacitively coupledquantumdots and a single-level dot attached to spin-polarized leads. Waiting time correlations are shown to give insight into the internal dynamics of the system; for example they allow distinction between different mechanisms of the noise enhancement. Moreover, the presence of correlations breaks the validity of the renewal theory. This increases the number of independent cumulants of current fluctuation statistics, thus providing additional sources of information about the transport mechanism. I also propose a method for inferring the presence of waiting time correlations based on low-order current correlation functions. This method gives a way to extend the analysis of nonrenewal current fluctuations to the systems for which single-electron counting is not experimentally feasible. The experimental relevance of the findings is also discussed; for example reanalysis of previous results concerning transport in quantumdots is suggested.

The study of supercrystals made of periodically arranged semiconductor quantumdots is essential for the advancement of emerging nanophotonics technologies. By combining the strong spatial confinement of elementary excitations inside quantumdots and exceptional design flexibility, quantum-dot supercrystals provide broad opportunities for engineering desired optical responses and developing superior light manipulation techniques on the nanoscale. Here we suggest tailoring the energy spectrum and wave functions of the supercrystals' collective excitations through the variation of different structural and material parameters. In particular, by calculating the excitonic spectra of quantumdots assembled in two-dimensional Bravais lattices we demonstrate a wide variety of spectrum transformation scenarios upon alterations in the quantumdot arrangement. This feature offers unprecedented control over the supercrystal's electromagnetic properties and enables the development of new nanophotonics materials and devices.

Semiconductor quantumdots are quickly becoming a critical diagnostic tool for discerning cellular function at the molecular level. Their high brightness, long-lasting, size-tunable, and narrow luminescence set them apart from conventional fluorescence dyes. Quantumdots are being developed for a variety of biologically oriented applications, including fluorescent assays for drug discovery, disease detection, single protein tracking, and intracellular reporting. This review introduces the science behind quantumdots and describes how they are made biologically compatible. Several applications are also included, illustrating strategies toward target specificity, and are followed by a discussion on the limitations of quantumdot approaches. The article is concluded with a look at the future direction of quantumdots.

The spin of a single electron subject to a static magnetic field provides a natural two-level system that is suitable for use as a quantum bit, the fundamental logical unit in a quantum computer. Semiconductor quantumdots fabricated by strain driven self-assembly are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned, electronically coupled and embedded into active devices. It has been predicted that the atomic-like electronic structure of such quantumdots suppresses coupling of the spin to the solid-state quantumdot environment, thus protecting the 'spin' quantum information against decoherence. Here we demonstrate a single electron spin memory device in which the electron spin can be programmed by frequency selective optical excitation. We use the device to prepare single electron spins in semiconductor quantumdots with a well defined orientation, and directly measure the intrinsic spin flip time and its dependence on magnetic field. A very long spin lifetime is obtained, with a lower limit of about 20 milliseconds at a magnetic field of 4 tesla and at 1 kelvin.

We demonstrate the development of a double quantumdot with an integrated charge sensor fabricated in undoped GaAs/AlGaAs heterostructures using a double top-gated design. Based on the evaluation of the integrated charge sensor, the double quantumdot can be tuned to a few-electron region. Additionally, the inter-dotcoupling of the double quantumdot can be tuned to a large extent according to the voltage on the middle gate. The quantumdot is shown to be tunable from a single dot to a well-isolated double dot. To assess the stability of such design, the potential fluctuation induced by 1/f noise was measured. Based on the findings herein, the quantumdot design developed in the undoped GaAs/AlGaAs semiconductor shows potential for the future exploitation of nano-devices.

We study magnetism in magnetically doped quantumdots as a function of particle numbers, temperature, confining potential, and the strength of Coulomb interaction screening. We show that magnetism can be tailored by controlling the electron-electron Coulomb interaction, even without changing the number of particles. The interplay of strong Coulomb interactions and quantum confinement leads to enhanced inhomogeneous magnetization which persists at substantially higher temperatures than in the non-interacting case or in the bulk-like dilute magnetic semiconductors. We predict a series of electronic spin transitions which arise from the competition between the many-body gap and magnetic thermal fluctuations. Cond-mat/0612489. [1] R. Abolfath, P. Hawrylak, I. Zuti'c, preprint.

We have calculated the transport properties of electron through an artificial quantumdot by using the numerical renormalization group technique in this paper.We obtain the conductance for the system of a quantumdot which is embedded in a one-dimensional chain in zero and finite temperature cases.The external magnetic field gives rise to a negative magnetoconductance in the zero temperature case.It increases as the external magnetic field increases.We obtain the relation between the coupling coefficient and conductance.If the interaction is big enough to prevent conduction electrons from tunnelling through the dot,the dispersion effect is dominant in this case.In the Kondo temperature regime,we obtain the conductivity of a quantumdot system with Kondo correlation.

We theoretically investigate the single- and few-electron ground-states properties of HgTe topological insulator quantumdots with rectangular hard-wall confining potential using configuration interaction method. For the case of single electron, the edge states is robust against the deformation from a square quantumdot to a rectangular ones, in contrast to the bulk states, the energy gap of the QDs increased due to the coupling of the opposite edge states; for the case of few electrons, the electrons first fill the edge states in the bulk band gap and the addition energy exhibit universal even-odd oscillation due to the shape-independent two-fold degeneracy of the edge states. The size of this edge shell can be controlled by tuning the dot size, shape or the bulk band gap via lateral or vertical electric gating respectively of the HgTe quantumdot.

The optical properties of semiconductor quantumdots are in many respects similar to those of atoms. Since quantumdots can be defined by state-of-the-art semiconductor technologies, they exhibit long-term stability and allow for well-controlled and efficient interactions with both optical and electrical fields. Resonant ps excitation of single quantumdot photodiodes leads to new classes of coherent optoelectronic functions and devices, which exhibit precise state preparation, phase-sensitive optical manipulations and the control of quantum states by electrical fields.

The optical properties of semiconductor quantumdots are in many respects similar to those of atoms. Since quantumdots can be defined by state-of-the-art semiconductor technologies, they exhibit long-term stability and allow for well-controlled and efficient interactions with both optical and electrical fields. Resonant ps excitation of single quantumdot photodiodes leads to new classes of coherent optoelectronic functions and devices, which exhibit precise state preparation, phase-sensitive optical manipulations and the control of quantum states by electrical fields.

. The main property of semiconductor quantumdots compared to bulk material or even quantum well structures is the discrete nature of the allowed states, which means that inversion of the medium can be obtained for very low electron densities. This has led to the fabrication of quantumdot lasers with record......-low threshold currents and amplifiers with record-high power levels. In this tutorial we will review the basic properties of quantumdots, emphasizing the properties which are important for laser and amplifier applications, as well as devices for all-optical signal processing. The high-speed properties...

Highlights: • The hyphenated system allows unequivocal identification of nanoparticle populations. • AF4 separation permitted detection of unexpected nanosized species in a sample. • ICP-QQQ provides elemental ratios with adequate accuracy in every nanoparticle. • Purity and chemical composition of different quantumdot samples were assessed. - Abstract: Separation and identification of nanoparticles of different composition, with similar particle diameter, coexisting in heterogeneous suspensions of polymer-coated CdSe/ZnS quantumdots (QDs) have been thoroughly assessed by asymmetric flow field-flow fractionation (AF4) coupled on-line to fluorescence and inductively coupled plasma mass spectrometry (ICPMS) detectors. Chemical characterization of any previously on-line separated nanosized species was achieved by the measurement of the elemental molar ratios of every element involved in the synthesis of the QDs, using inorganic standards and external calibration by flow injection analysis (FIA). Such elemental molar ratios, strongly limited so far to pure single nanoparticles suspensions, have been achieved with adequate accuracy by coupling for the first time an ICP-QQQ instrument to an AF4 system. This hyphenation turned out to be instrumental to assess the chemical composition of the different populations of nanoparticles coexisting in the relatively complex mixtures, due to its capabilities to detect the hardly detectable elements involved in the synthesis. Interestingly such information, complementary to that obtained by fluorescence, was very valuable to detect and identify unexpected nanosized species, present at significant level, produced during QDs synthesis and hardly detectable by standard approaches.

We study theoretically the return probability experiment, which is used to measure the dephasing time T-2*, in a double quantumdot (DQD) in semiconducting carbon nanotubes with spin-orbit coupling and disorder-induced valley mixing. Dephasing is due to hyperfine interaction with the spins of the C......-13 nuclei. Due to the valley and spin degrees of freedom, four bounded states exist for any given longitudinal mode in the quantumdot. At zero magnetic field, the spin-orbit coupling and the valley mixing split those four states into two Kramers doublets. The valley-mixing term for a given dot...... is determined by the intradot disorder; this leads to (i) states in the Kramers doublets belonging to different dots being different, and (ii) nonzero interdot tunneling amplitudes between states belonging to different doublets. We show that these amplitudes give rise to new avoided crossings, as a function...

When I was contacted by Kluwer Academic Publishers in the Fall of 200 I, inviting me to edit a volume of papers on the issue of electron transport in quantumdots, I was excited by what I saw as an ideal opportunity to provide an overview of a field of research that has made significant contributions in recent years, both to our understanding of fundamental physics, and to the development of novel nanoelectronic technologies. The need for such a volume seemed to be made more pressing by the fact that few comprehensive reviews of this topic have appeared in the literature, in spite of the vast activity in this area over the course of the last decade or so. With this motivation, I set out to try to compile a volume that would fairly reflect the wide range of opinions that has emerged in the study of electron transport in quantumdots. Indeed, there has been no effort on my part to ensure any consistency between the different chapters, since I would prefer that this volume instead serve as a useful forum for the...

Based on the highly sensitivity and stable-fluorescence of water-soluble CdTe/CdS core-shell quantumdots (QDs) with broad-specificity DNA aptamers, a novel ratiometric detection strategy was proposed for the sensitive detection of organophosphorus pesticides by capillary electrophoresis with laser-induced fluorescence (CE-LIF). The as-prepared QDs were first conjugated with the amino-modified oligonucleotide (AMO) by amidation reaction, which is partial complementary to the DNA aptamer of organophosphorus pesticides. Then QD-labeled AMO (QD-AMO) was incubated with the DNA aptamer to form QD-AMO-aptamer duplex. When the target organophosphorus pesticides were added, they could specifically bind the DNA aptamer, leading to the cleavage of QD-AMO-aptamer duplex, accompany with the release of QD-AMO. As a result, the ratio of peak height between QD-AMO and QD-AMO-aptamer duplex changed in the detection process of CE-LIF. This strategy was subsequently applied for the detection of phorate, profenofos, isocarbophos, and omethoate with the detection limits of 0.20, 0.10, 0.17, and 0.23μM, respectively. This is the first report about using QDs as the signal indicators for organophosphorus pesticides detection based on broad-specificity DNA aptamers by CE-LIF, thus contributing to extend the scope of application of QDs in different fields. The proposed method has great potential to be a universal strategy for rapid detection of aptamer-specific small molecule targets by simply changing the types of aptamer sequences.

This thesis reports research on quantumdotscoupled to dielectric and plasmonic nano-structures by way of nano-structure fabrication, optical measurements, and theoretical modeling. To study light-matter interaction, plasmonic gap waveguides with nanometer dimensions as well as samples for studies...... of quantumdots in proximity to semiconductor/air and semiconductor/metal interfaces, were fabricated. We measured the decay dynamics of quantumdots near plasmonic gap waveguides and observed modied decay rates. The obtainable modications with the fabricated structures are calculated to be too small...... for the spontaneous emission of mesoscopic quantum emitters is developed. The light-matter interaction is in this model modied beyond the dipole expectancy and found to both suppress and enhance the coupling to plasmonic modes in excellent agreement with our measurements. We demonstrate that this mesoscopic effect...

A temperature-dependent photoluminescence measurement is performed in CdSe/ZnSe quantumdots with a ZnCdSe quantum well. We deduce the temperature dependence of the exciton linewidth and peak energy of the zero-dimensional exciton in the quantumdots and two-dimensional exciton in the CdSe wetting layer. The experimental data reveal a reduction of homogeneous broadening of the exciton line in the quantumdots in comparison with that in the two-dimensional wetting layer, which indicates the decrease of exciton and optical phonon coupling in the CdSe quantumdots.

We show how an atomistic pseudopotential plus many-body configuration interaction theory can address the main spectroscopic features of self-assembled dots including, excitons, trions, biexcitons, fine-structure, charging spectra as well as electric-field dependence of entanglement in dot molecules.

We demonstrate the charge sensing of a few-donor double quantumdot precision placed with atomic resolution scanning tunnelling microscope lithography. We show that a tunnel-coupled single electron transistor (SET) can be used to detect electron transitions on both dots as well as inter-dot transitions. We demonstrate that we can control the tunnel times of the second dot to the SET island by ∼4 orders of magnitude by detuning its energy with respect to the first dot.

Full Text Available Luminescent semiconductor nanocrystals or quantumdots (QDs are a recentlydeveloped class of nanomaterial whose unique photophysical properties are helping tocreate a new generation of robust fluorescent biosensors. QD properties of interest forbiosensing include high quantum yields, broad absorption spectra coupled to narrow sizetunablephotoluminescent emissions and exceptional resistance to both photobleaching andchemical degradation. In this review, we examine the progress in adapting QDs for severalpredominantly in vitro biosensing applications including use in immunoassays, asgeneralized probes, in nucleic acid detection and fluorescence resonance energy transfer(FRET - based sensing. We also describe several important considerations when workingwith QDs mainly centered on the choice of material(s and appropriate strategies forattaching biomolecules to the QDs.

The coupling of semiconductor photocatalysts with graphene quantumdots (GQDs) has been proven to be an effective strategy to enhance the photocatalytic and photoelectrical conversion performances of the resulted composites; however, the preparation of semiconductor/GQDs composites usually involves several time-inefficient and tedious post-treatment steps. Herein, we present a facile one-step hydrothermal route for the preparation of GQDs coupled TiO2 (TiO2/GQDs) photocatalysts using 1,3,6-trinitropyrene (TNP) as the sole precursor of GQDs. During the hydrothermal process, TNP molecules undergo an intramolecular fusion to form GQDs, which simultaneously decorate on the surface of TiO2 nanoparticles, leading to a strong surface interaction between the two components. The effective coupling of GQDs on TiO2 can effectively extend the light absorption of the TiO2 to visible region and enhance the charge separation efficiency of TiO2/GQDs composites as a result of GQDs acting as a photosensitizer and an excellent electron acceptor. These key advances make the TiO2/GQDs photocatalyst highly active towards the H2 evolution reaction, resulting in 7 and 3 times higher H2 evolution rate and photocurrent response at optimal GQDs content than TiO2 alone, respectively. This study provides a new methodology for the development of high-performance GQDs modified semiconductor photocatalysts for energy conversion applications.

As a popular detection model, the fluorescence “turn-off” sensor based on quantumdots (QDs) has already been successfully employed in the detections of many materials, especially in the researches on the interactions between pesticides. However, the previous studies are mainly focused on simple single track or the comparison based on similar concentration of drugs. In this work, a new detection method based on the fluorescence “turn-off” model with water-soluble ZnCdSe and CdSe QDs simultaneously as the fluorescent probes is established to detect various pesticides. The fluorescence of the two QDs can be quenched by different pesticides with varying degrees, which leads to the differences in positions and intensities of two peaks. By combining with chemometrics methods, all the pesticides can be qualitative and quantitative respectively even in real samples with the limit of detection was 2 × 10{sup −8} mol L{sup −1} and a recognition rate of 100%. This work is, to the best of our knowledge, the first report on the detection of pesticides based on the fluorescence quenching phenomenon of double quantumdots combined with chemometrics methods. What's more, the excellent selectivity of the system has been verified in different mediums such as mixed ion disruption, waste water, tea and water extraction liquid drugs. - Highlights: • A new model based on double QDs is established for pesticide residues detection. • The fluorescent data array sensor is coupled with chmometrics methods. • The sensor can be highly sensitive and selective detection in actual samples.

We present a set of concrete and realistic ideas for the implementation of a small-scale quantum computer using electron spins in lateral GaAs/AlGaAs quantumdots. Initialization is based on leads in the quantum Hall regime with tunable spin-polarization. Read-out hinges on spin-to-charge conversion via spin-selective tunneling to or from the leads, followed by measurement of the number of electron charges on the dot via a charge detector. Single-qubit manipulation relies on a microfabricated wire located close to the quantumdot, and two-qubit interactions are controlled via the tunnel barrier connecting the respective quantumdots. Based on these ideas, we have begun a series of experiments in order to demonstrate unitary control and to measure the coherence time of individual electron spins in quantumdots.

In this thesis the thermoelectric properties (electrical conductance, Seebeck coefficient and thermal conductance)of quantumdots described by the Anderson impurity model have been investigated by using the numerical renormalization group (NRG) method. In order to make accurate calculations for thermoelectric properties of quantum impurity systems, a number of recent developments and refinements of the NRG have been implemented. These include the z-averaging and Campo discretization scheme, which enable the evaluation of physical quantities on an arbitrary temperature grid and at large discretization parameter Λ and the full density matrix (FDM) approach, which allows a more accurate calculation of spectral functions and transport coefficients. The implementation of the z-averaging and Campo discretization scheme has been tested within a new method for specific heats of quantum impurities. The accuracy of this new method was established by comparison with the numerical solution of the Bethe-ansatz equations for the Anderson model. The FDM approach was implemented and tested within a new approach to the calculation of impurity contributions to the uniform susceptibilities. Within this method a non-negligible contribution from the ''environmental'' degrees of freedom needs to be taken into account to recover the correct susceptibility, as shown by comparison with the Bethe-ansatz approach. An accurate method to calculate the conductance of a quantumdot is implemented, enabling the extraction of the Fermi liquid scaling coefficients c{sub T} and c{sub B} to high accuracy, being able to verify the results of the renormalized super perturbation theory approach (within its regime of validity). The method was generalized to higher order moments of the local level spectral function. This, as well as reduction of the SU(2) code to the U(1) symmetry, enabled the investigation of the effect of a magnetic field on the thermoelectric properties of quantum

The thermopower of few-electron quantumdots is crucially influenced by on-dot electron-electron interactions, particularly in the presence of Kondo correlations. We present a comprehensive picture which elucidates the underlying relations between the thermopower and the spectral density function of two-level quantumdots. The effects of various electronic states, including the Kondo states originating from both spin and orbital degrees of freedom, are clearly unraveled. With these insights, we have exemplified an effective and viable way to control the sign of thermopower of Kondo-correlated quantumdots. This is realized by tuning the temperature and by selecting the appropriate level spacing and Coulomb repulsion strength. Such a physical picture is affirmed by accurate numerical data obtained with a hierarchical equations of motion approach. Our understandings and findings provide useful insights into controlling the direction of electric (heat) current through a quantumdot by applying a temperature (voltage) gradient across the two coupling leads. This may have important implications for novel thermoelectric applications of quantumdots. The support from the Natural Science Foundation of China (Grants No. 21033008, No. 21233007, No. 21303175, and No. 21322305) and the Strategic Priority Research Program (B) of the CAS (XDB01020000) is gratefully appreciated.

Coherent coupling of two quantumdots embedded in a ring-geometry has been demonstrated by Holleitner et al.(A.W. Holleitner, H. Qin, C.R. Decker, K. Eberl, and R.H. Blick, phCoherent Coupling of Two QuantumDots Embedded in an Aharonov-Bohm Ring), Phys. Rev. Lett. 87, 256802 (2001) Recording of the Aharonov-Bohm oscillations in such a circuit has proven that the phases of electron wave functions can be manipulated directly. We have since enhanced the complexity of this system by embedding three quantumdots in such a ring-geometry. As before, our quantumdots are formed by laterally constricting a two-dimensional electron gas (2DEG) in an Al_xGa_1-xAs/GaAs heterostructure. The new, essential ingredient of this experiment is an additional third port--added to the ring for individually addressing the third quantumdot. This circuit allows us to investigate phenomena associated with phase-switching between separate ports. We will discuss first results and give a simple model of circuit operation.

We have measured the oscillator strength and quantum efficiency of excitons confined in large InGaAs quantumdots by recording the spontaneous emission decay rate while systematically varying the distance between the quantumdots and a semiconductor-air interface. The size of the quantumdots...... is measured by in-plane transmission electron microscopy and we find average in-plane diameters of 40 nm. We have calculated the oscillator strength of excitons of that size assuming a quantum-dot confinement given by a parabolic in-plane potential and a hard-wall vertical potential and predict a very large...... oscillator strength due to Coulomb effects. This is in stark contrast to the measured oscillator strength, which turns out to be so small that it can be described by excitons in the strong confinement regime. We attribute these findings to exciton localization in local potential minima arising from alloy...

Quantum-dot-molecular phase coherence (and the relevant quantum-interference-switchable optical response) can be utilized to control electromagnetic wave propagation via a gate voltage, since quantum-dot molecules can exhibit an effect of quantum coherence (phase coherence) when quantum-dot-molecular discrete multilevel transitions are driven by an electromagnetic wave. Interdot tunneling of carriers (electrons and holes) controlled by the gate voltage can lead to destructive quantum interference in a quantum-dot molecule that is coupled to an incident electromagnetic wave, and gives rise to a quantum coherence effect (e.g., electromagnetically induced transparency, EIT) in a quantum-dot-molecule dielectric film. The tunable on- and off-resonance tunneling effect of an incident electromagnetic wave (probe field) through such a quantum-coherent quantum-dot-molecule dielectric film is investigated. It is found that a high gate voltage can lead to the EIT phenomenon of the quantum-dot-molecular systems. Under the condition of on-resonance light tunneling through the present quantum-dot-molecule dielectric film, the probe field should propagate without loss if the probe frequency detuning is zero. Such an effect caused by both EIT and resonant tunneling, which is sensitive to the gate voltage, can be utilized for designing devices such as photonic switching, transistors, and logic gates.

We perform scanning gate microscopy on individual suspended carbon nanotube quantumdots. The size and position of the quantumdots can be visually identified from the concentric high conductance rings. For the ultra clean devices used in this study, two new effects are clearly identified. Electrostatic screening creates non-overlapping multiple sets of Coulomb rings from a single quantumdot. In double quantumdots, by changing the tip voltage, the interactions between the quantumdots can b...

Nanostructures made of semiconductors, such as quantum wells and quantumdots (QD), are well known, and some have been incorporated in practical devices. Here we focus on novel structures made of QDs and related devices for terahertz (THz) generation. Their potential advantages, such as low threshold current density, high characteristic temperature, increased differential gain, etc, make QDs promising candidates for light emitting applications in the THz region. Our idea of using resonant tunneling through QDs is presented, and initial results on devices consisting of self-assembled InAs QDs in an undoped GaAs matrix, with a design incorporating a GaInNAs/GaAs short period superlattice, are discussed. Moreover, shallow impurities are also being explored for possible THz emission: the idea is based on the tunneling through bound states of individual donor or acceptor impurities in the quantum well. Initial results on devices having an AlGaAs/GaAs double-barrier resonant tunneling structure are discussed.

The electronic spectral and transport properties of aperiodic quantumdot chains are investigated. The systems with singular continuous energy spectrum are considered: Thue-Morse chain, double-periodic chain, Rudin-Shapiro chain. The influence of electronic energy in quantumdot on the spectral properties, band structure, density of states and spectral resistivity, is discussed. Low resistivity regions correspond to delocalized states and these states could be current states. Also we discuss the magnetic field application as the way to tune electronic energy in quantumdot and to obtain metallic or insulating conducting states of the systems.

In this paper, we have realized amplification without inversion (AWI) in quantumdot (QD). A Y-type four-level system of InxGa1-xN quantumdot has been obtained and investigated for AWI. It has been shown that, with proper setting of control fields' amplitude, we can obtain reasonable gain. With proper setting of phase difference of control fields and probe field, we can obtain considerable gain in resonant wavelength. We have designed this system by solving the Schrödinger-Poisson equations for InxGa1-xN quantumdot in GaN substrate, self-consistently.

The desire to have a source of single entangled photon pairs can be satisfied using single quantumdots as emitters. However, we are not bound to pursue only polarization entanglement, but can also exploit other degrees of freedom. In this chapter we focus on the time degree of freedom, to achieve so-called time-bin entanglement. This requires that we prepare the quantumdot coherently into the biexciton state and also build special interferometers for analysis. Finally this technique can be extended to achieve time-bin and polarization hyper-entanglement from a suitable quantumdot.

Fluorescent semiconductor quantumdots that can serve as "on/off" labels for bacteria and other living cells are undergoing development. The "on/off" characterization of these quantumdots refers to the fact that, when properly designed and manufactured, they do not fluoresce until and unless they come into contact with viable cells of biological species that one seeks to detect. In comparison with prior fluorescence-based means of detecting biological species, fluorescent quantumdots show promise for greater speed, less complexity, greater sensitivity, and greater selectivity for species of interest. There are numerous potential applications in medicine, environmental monitoring, and detection of bioterrorism.

@@ The quantumdotcoupled to reservoirs is known as a typical mesoscopic setup to manifest the quantum characteristics of particles in transport. In analogue to many efforts made on the study of electronic quantumdots in the past decades, we study the transport of bosons through such a device. We first generalize the formula which relates the current to the local properties of dot in the bosonic situation. Then, as an illustrative example, we calculate the local density of state and lesser Green function of the localized boson with a bosonic Fano-Anderson model The current-voltage (Ⅰ-Ⅴ) behaviour at zero temperature is presented, and in the bosonic dot it is the Ⅰ-Ⅴ curve, in contrast to the differential conductance in the electronic dot, which is found to be proportional to the spectral function.

As a good luminescent material , quantumdots can be a probe for targets in complicated system through the surface modification with recognition function molecule .CdTe quantumdots ( QDs) were synthesized in aque-ous solution using mercaptoethylamine ( MA) as stabilizer .The optimal conditions for synthesis were studied by flu-orescence spectroscopy .The purification of CdTe QDs with different solvents and pH values of buffer solution was investigated systematically .The results showed that the purification with twice volume isopropanol was the best . The fluorescence of QDs basically unchanged in pH 7.4 buffer solution .The jasmonic acid-CdTe conjugates was obtained by coupling CdTe QDs with jasmonic acid ( JA) using the N-hydroxysuccinimide ( NHS) as a coupling a-gent.The optimal conditions for coupling reaction are pH 7.4 and stiring for 30 min at 37 ℃.After the JA mole-cule linked with CdTe QDs , the maximum absorption peak of jasmonic acid-CdTe conjugates was blue-shifted by 10 nm.The photostability of jasmonic acid-CdTe conjugates was also studied by fluorescence microscope .The jasmon-ic acid-CdTe conjugates was applied preliminarily in the labeling of root sections of mung bean seedlings .%以巯基乙胺（ MA）作为修饰剂，在水相条件下合成了稳定的CdTe量子点，优化了合成反应条件；其中用异丙醇做沉淀剂效果好，量子点重新分散到pH值为7.4的Tris-HCl缓冲溶液中，荧光基本保持不变；用N-羟基琥珀酰亚胺（NHS）作偶联剂，量子点与茉莉酸（JA）在pH值为7.4，37℃反应30 min合成了具有识别茉莉酸结合位点的探针． CdTe-JA最大吸收峰与CdTe相比蓝移了约10 nm，抗光漂白性好，并初步对绿豆幼苗根部进行了荧光标记．

In this report, we review the results of our joint experimental and theoretical studies of electron-interference, and interaction, phenomena in open electron cavities known as quantumdots. The transport through these structures is shown to be heavily influenced by the remnants of their discrete density of states, elements of which remain resolved in spite of the strong coupling that exists between the cavity and its reservoirs. The experimental signatures of this density of states are discussed at length in this report, and are shown to be related to characteristic wavefunction scarring, involving a small number of classical orbits. A semiclassical analysis of this behaviour shows it to be related to the effect of dynamical tunnelling, in which electrons are injected into the dot tunnel through classically forbidden regions of phase space, to access isolated regular orbits. The dynamical tunnelling gives rise to the formation of long-lived quasi-bound states in the open dots, and the many-body implications a...

This thesis explores the understanding of the chemistry and physics of colloidal quantumdots for practical solar energy photoconversion. Solar cell devices that make use of PbS quantumdots generally rely on constant and unchanged optical properties such that band gap energies remain tuned within the device. The design and development of unique experiments to ascertain mechanisms of optical band gap shifts occurring in PbS quantumdot thin-films exposed to air are discussed. The systematic s...

We propose an implementation of quantum neural networks using an array of quantumdots with dipole-dipole interactions. We demonstrate that this implementation is both feasible and versatile by studying it within the framework of GaAs based quantumdot qubits coupled to a reservoir of acoustic phonons. Using numerically exact Feynman integral calculations, we have found that the quantum coherence in our neural networks survive for over a hundred ps even at liquid nitrogen temperatures (77 K), which is three orders of magnitude higher than current implementations, which are based on SQUID-based systems operating at temperatures in the mK range.

We propose an implementation of quantum neural networks using an array of quantumdots with dipole-dipole interactions. We demonstrate that this implementation is both feasible and versatile by studying it within the framework of GaAs based quantumdot qubits coupled to a reservoir of acoustic phonons. Using numerically exact Feynman integral calculations, we have found that the quantum coherence in our neural networks survive for over a hundred ps even at liquid nitrogen temperatures (77 K), which is three orders of magnitude higher than current implementations, which are based on SQUID-based systems operating at temperatures in the mK range.

The research described in this thesis is aimed at constructing a quantum interface between a single electron spin and a photon, using a nanowire quantumdot. Such a quantum interface enables information transfer from a local electron spin to the polarization of a photon for long distance readout.

In this paper we study the implementation of non-adiabatic geometrical quantum gates with in semiconductor quantumdots. Different quantum information enconding/manipulation schemes exploiting excitonic degrees of freedom are discussed. By means of the Aharanov-Anandan geometrical phase one can avoid the limitations of adiabatic schemes relying on adiabatic Berry phase; fast geometrical quantum gates can be in principle implemented

Quantumdots are very attractive as fluorescent markers because of their excellent optical properties. For this reason, they have also been used to label liposomes by means of encapsulation, though their feasibility as liposome labels is often hampered by the presence of unencapsulated quantumdots. Until now, laborious gradient ultracentrifugation or less efficient size exclusion chromatography has been the methods of choice to remove unencapsulated quantumdots. Of these two strategies, size exclusion chromatography is most commonly used, despite the known poor separation. Consequently, this prompts for a choice between purification methods yielding high-purity quantumdot-loaded liposomes but low yields or vice versa. Herein, we present a novel high-yield and high-purity methodology to remove unencapsulated quantumdots in a quick and efficient manner based on electrostatic binding of quantumdots to ion-exchange beads. This was accomplished either by means of short column chromatography or via a simple pull-down approach. The purification efficiency was easily assessed via analytical gel electrophoresis, and by copper-mediated quenching of quantumdot fluorescence, it was established that the quantumdots were not adhered to the liposomes but encapsulated inside these. Furthermore, the recovery degree of quantumdot-loaded liposomes after ion-exchange purification was found to be excellent compared with size exclusion chromatography. Lastly, a method is presented to quantify the number of quantumdots encapsulated in the liposomes by the combined efforts of particle counting and inductively coupled plasma mass spectrometry.

Quantumdots are very attractive as fluorescent markers because of their excellent optical properties. For this reason, they have also been used to label liposomes by means of encapsulation, though their feasibility as liposome labels is often hampered by the presence of unencapsulated quantumdots. Until now, laborious gradient ultracentrifugation or less efficient size exclusion chromatography has been the methods of choice to remove unencapsulated quantumdots. Of these two strategies, size exclusion chromatography is most commonly used, despite the known poor separation. Consequently, this prompts for a choice between purification methods yielding high-purity quantumdot-loaded liposomes but low yields or vice versa. Herein, we present a novel high-yield and high-purity methodology to remove unencapsulated quantumdots in a quick and efficient manner based on electrostatic binding of quantumdots to ion-exchange beads. This was accomplished either by means of short column chromatography or via a simple pull- down approach. The purification efficiency was easily assessed via analytical gel electrophoresis, and by copper-mediated quenching of quantumdot fluorescence, it was established that the quantumdots were not adhered to the liposomes but encapsulated inside these. Furthermore, the recovery degree of quantumdot-loaded liposomes after ion-exchange purification was found to be excellent compared with size exclusion chromatography. Lastly, a method is presented to quantify the number of quantumdots encapsulated in the liposomes by the combined efforts of particle counting and inductively coupled plasma mass spectrometry.

In this study, we have calculated the unperturbed wavefunctions and energy eigenvalues of the ground and excited states of a spherical quantumdot, GaAs/AlxGa1-xAs, by using quantum genetic algorithm and Hartree-Fock Roothaan method. Hyperfine coupling constant and hyperfine energy of 1s, 2p, 3d and 4f levels are carried out as a function of dot radius. The results show that the hyperfine constant and hyperfine energy varies rapidly in the strong and medium confining regions as dot radius decreases. It is worth pointing out that dot radius, impurity charge and angular momentum have a strong influence on the hyperfine energy. It is also found that hyperfine energy and hyperfine splitting vary with aluminium concentration ratio x.

We provide a self-contained review of master equation approaches to modelling phonon effects in optically driven self-assembled quantumdots. Coupling of the (quasi) two-level excitonic system to phonons leads to dissipation and dephasing, the rates of which depend on the excitation conditions...

The NASA Glenn Research Center has been investigating the synthesis of quantumdots of CdSe and CuInS2 for use in intermediate-bandgap solar cells. Using quantumdots in a solar cell to create an intermediate band will allow the harvesting of a much larger portion of the available solar spectrum. Theoretical studies predict a potential efficiency of 63.2 percent, which is approximately a factor of 2 better than any state-of-the-art devices available today. This technology is also applicable to thin-film devices--where it offers a potential four-fold increase in power-to-weight ratio over the state of the art. Intermediate-bandgap solar cells require that quantumdots be sandwiched in an intrinsic region between the photovoltaic solar cell's ordinary p- and n-type regions (see the preceding figure). The quantumdots form the intermediate band of discrete states that allow sub-bandgap energies to be absorbed. However, when the current is extracted, it is limited by the bandgap, not the individual photon energies. The energy states of the quantumdot can be controlled by controlling the size of the dot. Ironically, the ground-state energy levels are inversely proportional to the size of the quantumdots. We have prepared a variety of quantumdots using the typical organometallic synthesis routes pioneered by Ba Wendi et al., in the early 1990's. The most studied quantumdots prepared by this method have been of CdSe. To produce these dots, researchers inject a syringe of the desired organometallic precursors into heated triocytlphosphine oxide (TOPO) that has been vigorously stirred under an inert atmosphere (see the following figure). The solution immediately begins to change from colorless to yellow, then orange and red/brown, as the quantumdots increase in size. When the desired size is reached, the heat is removed from the flask. Quantumdots of different sizes can be identified by placing them under a "black light" and observing the various color differences in

This paper presents an effective and feasible eigen-energy scanning method to solve polynomial matrix eigenvalues introduced by 3D quantumdots problem with band non-parabolicity. The pyramid-shaped quantumdot is placed in a computational box with uniform mesh in Cartesian coordinates. Its corresponding Schrödinger equation is discretized by the finite difference method. The interface conditions are incorporated into the discretization scheme without explicitly enforcing them. By comparing the eigenvalues from isolated quantumdots and a vertically aligned regular array of them, we investigate the coupling effect for variable distances between the quantumdots and different size.

Full Text Available The effect of geometry of the semimagnetic QuantumDot on the Interaction energy of a double acceptor is computed in the effective mass approximation using the variational principle. A peak is observed at the lower dot sizes as a magnetic field is increased which is attributed to the reduction in confinement.

We present low-temperature transport measurements on quantumdots induced in homogeneous InAs quantum wires 50 nm in diameter. Quantumdots are induced by electrical depletion of the wire using local gate electrodes with down to 30 nm electrode spacing. This scheme has permitted the realization of fully gate-defined multiple quantumdots along the nanowire [1]. Tunability in double quantumdots is a prerequisite for the system to be operated as a quantum gate. We demonstrate control over the lead tunnel barrier transparencies and, in the case of double quantumdots, the interdot coupling. Using the local gate electrodes also as plunger gates we measure double dot honeycomb stability diagrams which show the transition from a single large dot to two weakly coupleddots at 4.2K. The induced quantumdots can be tuned into the few-electron regime which is shown from Coulomb blockade measurements. We extract values of orbital energy-level spacings, capacitances and capacitive and tunnel interdot coupling for this system. [1] C. Fasth et al., NanoLett 5, 1487 (2005).

Quantumdots pose a problem where one must confront three obstacles: randomness, interactions and finite size. Yet it is this confluence that allows one to make some theoretical advances by invoking three theoretical tools: Random Matrix theory (RMT), the Renormalization Group (RG) and the 1/N expansion. Here the reader is introduced to these techniques and shown how they may be combined to answer a set of questions pertaining to quantumdots

We will here describe the results of the influence of electric field on InP quantumdots embedded in GalnP, lattice matched to GaAs. Experimental...details The sample we used was grown by metal-organic vapour phase epitaxy, and contained InP quantumdots in GanP, lattice matched to GaAs (n-type

Three different size distributions of Ge quantumdots (>~200, 110, and 60 Å) have been synthesized via the ultrasonic mediated reduction of mixtures of chlorogermanes and organochlorogermanes (or organochlorosilanes) by a colloidal sodium/potassium alloy in heptane, followed by annealing in a sealed pressure vessel at 270 °C. The quantumdots are characterized by transmission electron microscopy, x-ray powder diffraction, x-ray photoemission, infrared spectroscopy, and Raman spectroscopy. Col...

We report on strong renormalization encountered in periodically driven interacting quantumdots in the nonadiabatic regime. Correlations between lead and dot electrons enhance or suppress the amplitude of driving depending on the sign of the interaction. Employing a newly developed flexible renormalization-group-based approach for periodic driving to an interacting resonant level we show analytically that the magnitude of this effect follows a power law. Our setup can act as a non-Markovian, single-parameter quantum pump.

A method has been developed for inserting submonolayer (SML) quantumdots (QDs) or SML QD stacks, instead of conventional Stranski-Krastanov (S-K) QDs, into the active region of intersubband photodetectors. A typical configuration would be InAs SML QDs embedded in thin layers of GaAs, surrounded by AlGaAs barriers. Here, the GaAs and the AlGaAs have nearly the same lattice constant, while InAs has a larger lattice constant. In QD infrared photodetector, the important quantization directions are in the plane perpendicular to the normal incidence radiation. In-plane quantization is what enables the absorption of normal incidence radiation. The height of the S-K QD controls the positions of the quantized energy levels, but is not critically important to the desired normal incidence absorption properties. The SML QD or SML QD stack configurations give more control of the structure grown, retains normal incidence absorption properties, and decreases the strain build-up to allow thicker active layers for higher quantum efficiency.

This chapter provides a short review of quantumdots (QDs) physics, applications, and perspectives. The main advantage of QDs over bulk semiconductors is the fact that the size became a control parameter to tailor the optical properties of new materials. Size changes the confinement energy which alters the optical properties of the material, such as absorption, refractive index, and emission bands. Therefore, by using QDs one can make several kinds of optical devices. One of these devices transforms electrons into photons to apply them as active optical components in illumination and displays. Other devices enable the transformation of photons into electrons to produce QDs solar cells or photodetectors. At the biomedical interface, the application of QDs, which is the most important aspect in this book, is based on fluorescence, which essentially transforms photons into photons of different wavelengths. This chapter introduces important parameters for QDs' biophotonic applications such as photostability, excitation and emission profiles, and quantum efficiency. We also present the perspectives for the use of QDs in fluorescence lifetime imaging (FLIM) and Förster resonance energy transfer (FRET), so useful in modern microscopy, and how to take advantage of the usually unwanted blinking effect to perform super-resolution microscopy.

We investigate the influence of electron-phonon interactions on the dynamical properties of a quantum-dot-cavity QED system. We show that non-Markovian effects in the phonon reservoir lead to strong changes in the dynamics, arising from photon-assisted dephasing processes, not present in Markovian...

A group of quantumdots can be designed to have a unique spectral emission by varying the size of the quantumdots (wavelength) and number of quantumdots (intensity). This technique has been previously proposed for biological tags and object identification. The potential of this system lies in the ability to have a large number of distinguishable wavelengths and intensity levels. This paper presents a communications system model for MxQDs including the interference between neighbouring QD colours and detector noise. An analytical model of the signal-to-noise ratio of a Charge-Coupled Device (CCD) spectrometer is presented and confirmed with experimental results. We then apply a communications system perspective and propose data detection algorithms that increase the readability of the quantumdots tags. It is demonstrated that multiplexed quantumdot barcodes can be read with 99.7% accuracy using the proposed data detection algorithms in a system with 6 colours and 6 intensity values resulting in 46,655 unique spectral codes.

We present the Quantum Computer Aided Design (QCAD) simulator that targets modeling quantum devices, particularly silicon double quantumdots (DQDs) developed for quantum qubits. The simulator has three di erentiating features: (i) its core contains nonlinear Poisson, e ective mass Schrodinger, and Con guration Interaction solvers that have massively parallel capability for high simulation throughput, and can be run individually or combined self-consistently for 1D/2D/3D quantum devices; (ii) the core solvers show superior convergence even at near-zero-Kelvin temperatures, which is critical for modeling quantum computing devices; (iii) it couples with an optimization engine Dakota that enables optimization of gate voltages in DQDs for multiple desired targets. The Poisson solver includes Maxwell- Boltzmann and Fermi-Dirac statistics, supports Dirichlet, Neumann, interface charge, and Robin boundary conditions, and includes the e ect of dopant incomplete ionization. The solver has shown robust nonlinear convergence even in the milli-Kelvin temperature range, and has been extensively used to quickly obtain the semiclassical electrostatic potential in DQD devices. The self-consistent Schrodinger-Poisson solver has achieved robust and monotonic convergence behavior for 1D/2D/3D quantum devices at very low temperatures by using a predictor-correct iteration scheme. The QCAD simulator enables the calculation of dot-to-gate capacitances, and comparison with experiment and between solvers. It is observed that computed capacitances are in the right ballpark when compared to experiment, and quantum con nement increases capacitance when the number of electrons is xed in a quantumdot. In addition, the coupling of QCAD with Dakota allows to rapidly identify which device layouts are more likely leading to few-electron quantumdots. Very efficient QCAD simulations on a large number of fabricated and proposed Si DQDs have made it possible to provide fast feedback for design

We report on the realization of few-electron double quantumdots defined in a two-dimensional electron gas by means of surface gates on top of a GaAs/AlGaAs heterostructure. Two quantum point contacts (QPCs) are placed in the vicinity of the double quantumdot and serve as charge detectors. These enable determination of the number of conduction electrons on each dot. This number can be reduced to zero, while still allowing transport measurements through the double dot. The coupling between the two dots can be controlled even in the few-electron regime. Microwave radiation is used to pump an electron from one dot to the other by absorption of a single photon. The experiments demonstrate that this quantumdot circuit can serve as a good starting point for a scalable spin-qubit system.

In this paper, we study the implementation of Grover's algorithm using the system of three identical quantumdots (QDs) coupled by a multi-frequency optical field. Our result shows that increasing the electric field strength A speeds up the oscillations of the occupations of the excited states rather than increasing the occupation probabilities of those states. The larger the detuning of the field from resonance, the fewer the states which can be used as qubits. Compared with a multi-frequency external field, a single-frequency external field will generate much lower amplitudes of the excited states under the same coupling strength A and interdot Coulomb interaction V. However, when the three quantumdots are coupled with a single-frequency external field, these amplitudes increase on increasing the coupling strength A or decreasing the interdot Coulomb interaction V.

We study the thermoelectric transport through a double-quantum-dot system with spin-dependent interdot coupling and ferromagnetic electrodes by means of the non-equilibrium Green's function in the linear response regime.It is found that the thermoelectric coefficients are strongly dependent on the splitting of the interdot coupling,the relative magnetic configurations,and the spin polarization of leads.In particular,the thermoelectric efficiency can reach a considerable value in the parallel configuration when the effective interdot coupling and the tunnel coupling between the quantumdots and the leads for the spin-down electrons are small.Moreover,the thermoelectric efficiency increases with the intradot Coulomb interaction increasing and can reach very high values at appropriate temperatures.In the presence of the magnetic field,the spin accumulation in the leads strongly suppresses the thermoelectric efficiency,and a pure spin thermopower can be obtained.

We study analytically the properties of the optical absorption and the spatial weak-light solitons in a quantumdot molecule system with the interdot tunneling coupling (ITC). It is shown that, for the linear case, there exists tunneling induced transparency (TIT) in the context of a weak ITC, while the TIT can be replaced by Autler-Townes splitting in the presence of a strong ITC. For the nonlinear case, it is probable to realize the spatial optical solitons even under weak light intensity. Interestingly, we find that there appears transformation behavior between the bright and dark solitons by properly turning both the ITC strength and the detuning of the probe field. Meanwhile, the transformation condition of the bright and dark solitons is obtained. Additionally it is also found that the amplitude of the solitons first descends and then rises with the increasing of ITC strength. Our results may have potential applications for nonlinear optical experiments and optical telecommunication engineering in solid systems.

Quantumdots are defined as very small semiconductor crystals of size varying from nanometer scale to a few micron i.e. so small that they are considered dimensionless and are capable of showing many chemical properties by virtue of which they tend to be lead at one minute and gold at the second minute.Quantumdots house the electrons just the way the electrons would have been present in an atom, by applying a voltage. And therefore they are very judiciously given the name of being called as the artificial atoms. This application of voltage may also lead to the modification of the chemical nature of the material anytime it is desired, resulting in lead at one minute to gold at the other minute. But this method is quite beyond our reach. A quantumdot is basically a semiconductor of very tiny size and this special phenomenon of quantumdot, causes the band of energies to change into discrete energy levels. Band gaps and the related energy depend on the relationship between the size of the crystal and the exciton radius. The height and energy between different energy levels varies inversely with the size of the quantumdot. The smaller the quantumdot, the higher is the energy possessed by it.There are many applications of the quantumdots e.g. they are very wisely applied to:Light emitting diodes: LEDs eg. White LEDs, Photovoltaic devices: solar cells, Memory elements, Biology : =biosensors, imaging, Lasers, Quantum computation, Flat-panel displays, Photodetectors, Life sciences and so on and so forth.The nanometer sized particles are able to display any chosen colour in the entire ultraviolet visible spectrum through a small change in their size or composition.

Strongly-coupledquantumdot molecules (QDMs) are widely employed in the design of a variety of optoelectronic, photovoltaic, and quantum information devices. An efficient and optimized performance of these devices demands engineering of the electronic and optical properties of the underlying QDMs. The application of electric fields offers a way to realise such a control over the QDM characteristics for a desired device operation. We performed multi-million-atom atomistic tight-binding calculations to study the influence of electric fields on the electron and hole wave function confinements and symmetries, the ground-state transition energies, the band-gap wavelengths, and the optical transition modes. Electrical fields parallel () and anti-parallel () to the growth direction were investigated to provide a comprehensive guide for understanding the electric field effects. The strain-induced asymmetry of the hybridized electron states is found to be weak and can be balanced by applying a small electric field, of the order of 1 kV cm-1. The strong interdot couplings completely break down at large electric fields, leading to single QD states confined at the opposite edges of the QDM. This mimics a transformation from a type-I band structure to a type-II band structure for the QDMs, which is a critical requirement for the design of intermediate-band solar cells (IBSCs). The analysis of the field-dependent ground-state transition energies reveals that the QDM can be operated both as a high dipole moment device by applying large electric fields and as a high polarizability device under the application of small electric field magnitudes. The quantum confined Stark effect (QCSE) red shifts the band-gap wavelength to 1.3 μm at the 15 kV cm-1 electric field; however the reduced electron-hole wave function overlaps lead to a decrease in the interband optical transition strengths by roughly three orders of magnitude. The study of the polarisation-resolved optical modes

Semiconducting quantumdots, whose particle sizes are in the nanometer range, have very unusual properties. The quantumdots have band gaps that depend in a complicated fashion upon a number of factors, described in the article. Processing-structure-properties-performance relationships are reviewed for compound semiconducting quantumdots. Various methods for synthesizing these quantumdots are discussed, as well as their resulting properties. Quantum states and confinement of their excitons ...

: the predominance of spin-zero nuclei suppresses the hyperfine interaction and chemical synthesis creates a clean and defect-free system with highly controllable properties. Here we present a top gate-defined double quantumdot based on Ge/Si heterostructure nanowires with fully tunable coupling between the dots......Coupled electron spins in semiconductor double quantumdots hold promise as the basis for solid-state qubits. To date, most experiments have used III-V materials, in which coherence is limited by hyperfine interactions. Ge/Si heterostructure nanowires seem ideally suited to overcome this limitation...... and to the leads. We also demonstrate a novel approach to charge sensing in a one-dimensional nanostructure by capacitively coupling the double dot to a single dot on an adjacent nanowire. The double quantumdot and integrated charge sensor serve as an essential building block required to form a solid-state spin...

Phonon modes of A1As/GaAs/A1As and GaAs/A1As/metal Pb quantum-dotquantum wells (QDQW's) with the whole scale up to 90 AО are calculated by using valence force field model (VFFM) based on group theory.Their optical frequency spectra are divided into two nonoverlapping bands, the AlAs-like band and the GaAs-like band,originated from and having frequency interval inside the bulk AlAs optical band and bulk GaAs optical band, respectively.The GaAs-LO (Г)-like modes of QDQW's that have maximum bulk GaAs-LO (Г) parentages in all modes covering thewhole frequency region and all symmetries have always A1 symmetry. Its frequency is controllable by adjusting thestructure parameters. In A1As/GaAs/A1As, it may be controlled to meet any designed frequency in GaAs-like band.The results on GaAs/A1As/metal Pb QDQW's show the same effect of reducing in interface optical phonons by using the metal/semiconductor interface revealed ever by macroscopic model The frequency spectra in both GaAs-like andAlAs-like optical phonon bands are independent of the thickness of Pb shell as long as the thickness of Pb shell is no less than 5 AО Defects at metal/A1As interface have significant influence to AlAs-like optical modes but have only minor influence to GaAs-like optical modes. All these results are important for the studying of the e-ph interaction in QD structures.

Phonon modes of AlAs/GaAs/AlAs and GaAs/AlAs/metal Pb quantum-dotquantum wells (QDQW's)with the whole scale up to 90 A are calculated by using valence force field model (VFFM) based on group theory.Their optical frequency spectra are divided into two nonoverlapping bands, the AMs-like band and the GaAs-like band,originated from and having frequency interval inside the bulk AlAs optical band and bulk GaAs optical band, respectively.The GaAs-LO (F)-like modes of QDQW's that have maximum bulk GaAs-LO (F) parentages in all modes covering the whole frequency region and all symmetries have always A1 symmetry. Its frequency is controllable by adjusting the structure parameters. In AlAs/GaAs/AlAs, it may be controlled to meet any designed frequency in GaAs-like band.The results on GaAs/AMs/metal Pb QDQW's show the same effect of reducing in interface optical phonons by using the metal/semiconductor interface revealed ever by macroscopic model. The frequency spectra in both GaAs-like and AlAs-like optical phonon bands are independent of the thickness of Pb shell as long as the thickness of Pb shell is no less than 5 A. Defects at metal/AlAs interface have significant influence to AMs-like optical modes but have only minor influence to GaAs-like optical modes. All these results are important for the studying of the e-ph interaction in QD structures.

This thesis describes a series of experiments aimed at understanding and controlling the behavior of the spin degree of freedom of single electrons, confined in semiconductor quantumdots. This research work is motivated by the prospects of using the electron spin as a quantum bit (qubit), the basic

In this paper, we modeled a quantumdot at near proximity to a gap plasmon waveguide to study the quantumdot-plasmon interactions. Assuming that the waveguide is single mode, this paper is concerned about the dependence of spontaneous emission rate of the quantumdot on waveguide dimensions such as width and height. We compare coupling efficiency of a gap waveguide with symmetric configuration and asymmetric configuration illustrating that symmetric waveguide has a better coupling efficiency to the quantumdot. We also demonstrate that optimally placed quantumdot near a symmetric waveguide with 50 nm × 50 nm cross section can capture 80% of the spontaneous emission into a guided plasmon mode.

We study the conductance through a triangular triple quantumdot, which is connected to two noninteracting leads, using the numerical renormalization group (NRG). It is found that the system shows a variety of Kondo effects depending on the filling of the triangle. The SU(4) Kondo effect occurs at half-filling, and a sharp conductance dip due to a phase lapse appears in the gate-voltage dependence. Furthermore, when four electrons occupy the three sites on average, a local S=1 moment, which is caused by the Nagaoka mechanism, is induced along the triangle. The temperature dependence of the entropy and spin susceptibility of the triangle shows that this moment is screened by the conduction electrons via two separate stages at different temperatures. The two-terminal and four-terminal conductances show a clear difference at the gate voltages, where the SU(4) or the S=1 Kondo effects occur[1]. We will also discuss effects of deformations of the triangular configuration, caused by the inhomogeneity in the inter-dotcouplings and in the gate voltages. [4pt] [1] T.Numata, Y.Nisikawa, A.Oguri, and A.C.Hewson: arXiv:0808.3496.

We propose an efficient method to create multipartite spin entangled states in quantumdotscoupled to a nano electro-mechanical resonator array. Our method, based on the interaction between electron spins confined in quantumdots and the motion of magnetized nano electro-mechanical resonators, can enable a coherent spin-spin coupling over long distances and in principle be applied to an arbitrarily large number of electronic spins.%@@ We propose an efficient method to create multipartite spin entangled states in quantumdotscoupled to a nano electro-mechanical resonator array.Our method, based on the interaction between electron spins confined in quantumdots and the motion of magnetized nano electro-mechanical resonators, can enable a coherent spin-spin coupling over long distances and in principle be applied to an arbitrarily large number of electronic spins.

We consider two modifications of a recently proposed three-terminal quantumdot heat engine. First, we investigate the necessity of the thermalization assumption, namely that electrons are always thermalized by inelastic processes when traveling across the cavity where the heat is supplied. Second, we analyze various arrangements of tunneling-coupledquantumdots in order to implement a transmission function that is superior to the Lorentzian transmission function of a single quantumdot. We show that the maximum power of the heat engine can be improved by about a factor of two, even for a small number of dots, by choosing an optimal structure.

Dot-in-well (DWELL) quantum-dot infrared photodetectors (QDIPs) [DWELL-QDIPs] are subjects of research as potentially superior alternatives to prior QDIPs. Heretofore, there has not existed a reliable method for fabricating quantumdots (QDs) having precise, repeatable dimensions. This lack has constituted an obstacle to the development of uniform, high-performance, wavelength-tailorable QDIPs and of focal-plane arrays (FPAs) of such QDIPs. However, techniques for fabricating quantum-well infrared photodetectors (QWIPs) having multiple-quantum- well (MQW) structures are now well established. In the present research on DWELL-QDIPs, the arts of fabrication of QDs and QWIPs are combined with a view toward overcoming the deficiencies of prior QDIPs. The longer-term goal is to develop focal-plane arrays of radiationhard, highly uniform arrays of QDIPs that would exhibit high performance at wavelengths from 8 to 15 m when operated at temperatures between 150 and 200 K. Increasing quantum efficiency is the key to the development of competitive QDIP-based FPAs. Quantum efficiency can be increased by increasing the density of QDs and by enhancing infrared absorption in QD-containing material. QDIPs demonstrated thus far have consisted, variously, of InAs islands on GaAs or InAs islands in InGaAs/GaAs wells. These QDIPs have exhibited low quantum efficiencies because the numbers of QD layers (and, hence, the areal densities of QDs) have been small typically five layers in each QDIP. The number of QD layers in such a device must be thus limited to prevent the aggregation of strain in the InAs/InGaAs/GaAs non-lattice- matched material system. The approach being followed in the DWELL-QDIP research is to embed In- GaAs QDs in GaAs/AlGaAs multi-quantum- well (MQW) structures (see figure). This material system can accommodate a large number of QD layers without excessive lattice-mismatch strain and the associated degradation of photodetection properties. Hence, this material

Quantumdot sensitized solar cells (QDSCs) have attracted significant attention as promising third-generation photovoltaic devices. In the form of quantumdots (QDs), the semiconductor sensitizers have very useful and often tunable properties; moreover, their theoretical thermodynamic efficiency might be as high as 44%, better than the original 31% calculated ceiling. Unfortunately, the practical performance of these devices still lags behind that of dye-sensitized solar cells. In this Account, we summarize the strategies for depositing CdSe quantumdots on nanostructured mesoporous TiO(2) electrodes and discuss the methods that facilitate improvement in the performance and stability of QDSCs. One particularly significant factor for solar cells that use polysulfide electrolyte as the redox couple, which provides the best performance among QDSCs, is the passivation of the photoanode surface with a ZnS coating, which leads to a dramatic increase of photocurrents and efficiencies. However, these solar cells usually show a poor current-potential characteristic, so a general investigation of the recombination mechanisms is required for improvements. A physical model based on recombination through a monoenergetic TiO(2) surface state that takes into account the effect of the surface coverage has been developed to better understand the recombination mechanisms of QDSCs. The three main methods of QD adsorption on TiO(2) are (i) in situ growth of QDs by chemical bath deposition (CBD), (ii) deposition of presynthesized colloidal QDs by direct adsorption (DA), and (iii) deposition of presynthesized colloidal QDs by linker-assisted adsorption (LA). A systematic investigation by impedance spectroscopy of QDSCs prepared by these methods showed a decrease in the charge-transfer resistance and increased electron lifetimes for CBD samples; the same result was found after ZnS coating because of the covering of the TiO(2) surface. The increase of the lifetime with the ZnS treatment

We study an array of graphene nano sheets that form a two-dimensional S =1/2 Kagome spin lattice used for quantum computation.The edge states of the graphene nano sheets axe used to form quantumdots to confine electrons and perform the computation.We propose two schemes of bang-bang control to combat decoherence and realize gate operations on this array of quantumdots.It is shown that both schemes contain a great amount of information for quantum computation.The corresponding gate operations are also proposed.

Colloidal quantumdots are free-standing nanostructures with chemically tunable electronic properties. In this work, we consider a new STM tip-double quantumdot (DQD)-surface setup with a unique connectivity, in which the tip is coupled to a single dot and the coupling to the surface is shared by both dots. Our theoretical analysis reveals a unique negative differential resistance (NDR) effect attributed to destructive interference during charge transfer from the DQD to the surface. This NDR can be used as a sensitive probe for interdot interactions in DQD arrays.

We investigate the effects of the coupling between quantized vibrational modes (vibrons) and electronic degrees of freedom in suspended carbon nanotube quantumdots. The elastic deformations couple both to the total dot charge and to its spatial density fluctuations. The latter, neglected in the Anderson-Holstein model, have profound consequences if the size of the vibron and of the dot do not coincide, as is generically the case in experimental devices. When the vibron is smaller than the quantumdot, spatially varying local Franck-Condon factors emerge with striking effects on the transport properties of the system. The theoretical results are supported by our experimental observations.

In an asymmetry quantumdot, the properties of the electron, which is strongly coupled with phonon, were investigated. The variational relations of the first internal excited state energy, the excitation energy and the frequency of transition spectral line between the first internal excited state and the ground state of the electron which is strongly coupled with phonon in an asymmetry quantumdot with the transverse and longituainal effective confinement length of quantumdot and the electron-phonon coupling strength were studied by using a linear combination operator and the unitary transformation methods. Numerical calculations for the variational relations of the first internal excited state energy, the excitation energy and the frequency of transition spectral line between the first internal excited state and the ground state of the electron which is strongly coupled with phonon in an asymmetry quantumdot with the transverse and longituainal effective confinement length of quantumdot and the electron-phonon coupling strength were performed and the results show that the first internal excited state energy, the excitation energy and the frequency of transition spectral line between the first internal excited state and the ground state of the electron which is strongly coupled with phonon in an asymmetry quantumdot will strongly increase with decreasing the transverse and longitudinal effective confinement length. The first internal excited state energy of the electron which is strongly coupled with phonon in an asymmetry quantumdot will decrease with increasing the electron-phonon coupling strength. The excitation energy and the frequency of transition spectral line between the first internal excited state and the ground state of the electron which is strongly coupled with phonon in an asymmetry quantumdot will increase with increasing the electron-phonon coupling strength.

We investigate the conductance spectra of coupledquantumdots to study systematically the nuclear spin relaxation of delta- and y-junction networks and observe spin blockade dependence on the electronic configurations. We derive the conductance using the Beenakker approach generalised to an array of quantumdots where we consider the nuclear spin transfer to electrons by hyperfine coupling. This allows us to predict the relevant memory effects on the different electronic states by studying the evolution of the single electron resonances in presence of nuclear spin relaxation. We find that the gradual depolarisation of the nuclear system is imprinted in the conductance spectra of the multidot system. Our calculations of the temporal evolution of the conductance resonance reveal that spin blockade can be lifted by hyperfine coupling.

We present detuning-dependent spectral and decay-rate measurements to study the difference between the spectral and dynamical properties of single quantumdots embedded in micropillar and photonic crystal cavities. For the micropillar cavity, the dynamics is well described by the dissipative Jaynes......–Cummings model, whereas systematic deviations are observed for the emission spectra. The discrepancy for the spectra is attributed to the coupling of other exciton lines to the cavity and interference of different propagation paths toward the detector of the fields emitted by the quantumdot. In contrast......, quantitative information about the system can readily be extracted from the dynamical measurements. In the case of photonic crystal cavities, we observe an anti-crossing in the spectra when detuning a single quantumdot through resonance, which is the spectral signature of a strong coupling. However, time...

We explore exchange coupling of a pair of spins in a double dot and in an optical lattice, using the frequency of exchanges in a bosonic path integral, evaluated using Monte Carlo simulation. The algorithm gives insights into the role of correlation through visualization of two-particle probability...

We show that strong superconducting correlations can be induced in an interacting quantumdot (QD) using fast oscillations in the effective coupling between the dot and superconducting leads which drive the dot out of equilibrium. This is in contrast with the well-known equilibrium state suppression of proximity effect in interacting QDs. In fact although interaction prohibits the superposition of empty (0) and doubly-occupied (d) states, fast coherent dynamics accompanied by the fast variations in the tunnel coupling can produce a nonequilibrium finite probability for such a superposition. Subsequently the superconducting correlations are established inside the QD when the energy difference between 0 and d states coincide with the frequency of driving oscillations. Simultaneously the nonequilibrium occupation probabilities of 0 and d states cause a pumping current flowing to the normal lead connected to the dot. Finally we demonstrate coherent oscillations in both dot charge and current by applying a pulsed oscillatory field to the coupling of dot and superconductor which show the possibility of coherent manipulation in the subspace of 0 and d states by changing the pulse duration.

We demonstrate a single-photon collection efficiency of $(44.3\\pm2.1)\\%$ from a quantumdot in a low-Q mode of a photonic-crystal cavity with a single-photon purity of $g^{(2)}(0)=(4\\pm5)\\%$ and directly detect up to $962\\pm46$ kilocounts per second on a single-photon detector. The high collection efficiency is found to be broadband, as is confirmed by detailed numerical simulations. Cavity-enhanced efficient excitation of quantumdots is obtained through phonon-mediated excitation and under these conditions, single-photon indistinguishability measurements reveal long coherence times of up to $0.77\\pm0.19$ ns. Our work demonstrates that photonic crystals provide a very promising platform for highly integrated generation of coherent single photons including the efficient outcoupling of the photons from the photonic chip.

We discuss strategies to reduce midgap trap state densities in colloidal quantumdot films and requirements to control doping type and magnitude. We demonstrate that these improvements result in colloidal quantumdot solar cells with certified 7.0% efficiency.

Binding energies of a hydrogenic donor in a spherical GaAs quantumdot surrounded by Ga1-xAlxAs matrix are calculated. The results are presented for realistic barrier heights corresponding to different values of x (x < 0.4). The calculations are performed under two different conditions: (i) a spherical dot with square well confinement and (ii) a dot with parabolic potential well confinement. The results show that (i) the donor ionization energies are always higher under parabolic confinement as compared to a dot of the same radius under square well confinement and (ii) the oscillator strengths coupling ground state with excited states are two orders larger under parabolic confinement. Our results are in agreement with the results of other researchers.

We present a new scheme for quantum interfaces (QIs) to accomplish the interconversion of photonic qubits and spin qubits based on optomechanical resonators and the spin-orbit-induced interactions in suspended carbon nanotube quantumdots (CNTQDs). This interface implements quantum spin transducers and further enables electrical manipulation of local electron spin qubits, which lays the foundation for all-electrical control of state transfer protocols between two distant quantum nodes in a quantum network. We numerically evaluate the state transfer processes and proceed to estimate the effect of each coupling strength on the operation fidelities. The simulation suggests that high operation fidelities are obtainable under realistic experimental conditions.

Motivated by recent experiments on the generation of coherent light in engineered hybrid quantum systems, we investigate gain in a microwave photonic cavity coupled to quantumdot structures, and develop concrete directions for achieving a giant amplification in photon transmission. We propose two architectures for scaling up the electronic gain medium: (i) $N$ double quantumdot systems (N-DQD), (ii) $M$ quantumdots arranged in series akin to a quantum cascade laser setup. In both setups, the fermionic reservoirs are voltage biased, and the quantumdots are coupled to a single-mode cavity. Optical amplification is explained based on a sum rule for the transmission function, and it is determined by an intricate competition between two different processes: charge density response in the gain medium, and cavity losses to input and output ports. The same design principle is also responsible for the corresponding giant amplification in other photonic observables, mean photon number and emission spectrum, thereby...

We measured dephasing times in InGaAl/As self-assembled quantumdots at low temperature using degenerate four-wave mixing. At 0K, the coherence time of the quantumdots is lifetime limited, whereas at finite temperatures pure dephasing by exciton-phonon interactions governs the quantumdot...... coherence. The inferred homogeneous line widths are significantly smaller than the line widths usually observed in the photoluminescence from single quantumdots indicating an additional inhomogeneours broadening mechanism in the latter....

The modulation response of quantum-dot based nanocavity devices is investigated using a semiconductor theory. We show that high modulation bandwidth is achieved even in the presence of inhomogeneous broadening of the quantumdot ensemble.......The modulation response of quantum-dot based nanocavity devices is investigated using a semiconductor theory. We show that high modulation bandwidth is achieved even in the presence of inhomogeneous broadening of the quantumdot ensemble....

The present invention is directed to a method of preparing a carbon quantumdot. The carbon quantumdot can be prepared from a carbon precursor, such as a fullerene, and a complex metal hydride. The present invention also discloses a carbon quantumdot made by reacting a carbon precursor with a complex metal hydride and a polymer containing a carbon quantumdot made by reacting a carbon precursor with a complex metal hydride.

Cadmium sulfide quantumdots are perspective materials in optics, medicine, biology and optoelectronics. Fluorophosphate glasses, doped with cadmium sulfide quantumdots, were examined in the paper. Heat treatment led to the formation of quantumdots with diameters equal to 2.8 nm, 3.0 nm and 3.8 nm. In view of such changes in the quantumdots size the fundamental absorption edge shift and the luminescence band are being displaced to the long wavelengths. Luminescence lifetime has been fou...

We present a numerical study of the emergence of Majorana and Andreev bound states in a system composed of two quantumdots, one of which is coupled to a conventional superconductor, SC1, and the other connects to a topological superconductor, SC2. By controlling the interdot coupling we can drive the system from two single (uncoupled) quantumdots to double (coupled) dot system configurations. We employ a recursive Green’s function technique that provides us with numerically exact results for the local density of states of the system. We first show that in the uncoupled dot configuration (single dot behavior) the Majorana and the Andreev bound states appear in an individual dot in two completely distinct regimes. Therefore, they cannot coexist in the single quantumdot system. We then study the coexistence of these states in the coupled double dot configuration. In this situation we show that in the trivial phase of SC2, the Andreev states are bound to an individual quantumdot in the atomic regime (weak interdot coupling) or extended over the entire molecule in the molecular regime (strong interdot coupling). More interesting features are actually seen in the topological phase of SC2. In this case, in the atomic limit, the Andreev states appear bound to one of the quantumdots while a Majorana zero mode appears in the other one. In the molecular regime, on the other hand, the Andreev bound states take over the entire molecule while the Majorana state remains always bound to one of the quantumdots.

To implement high quality tunneling injection quantumdot lasers,effects of primary factors on performance of the tunneling injection quantumdot lasers were investigated. The considered factors were tunneling probability,tunneling time and carriers thermal escape time from the quantum well. The calculation results show that with increasing of the ground-state energy level in quantum well,the tunneling probability increases and the tunneling time decreases,while the thermal escape time decreases because the ground-state energy levelis shallower. Longitudinal optical phonon-assisted tunneling can be an effective method to solve the problem that both the tunneling time and the thermal escape time decrease simultaneously with the ground-state energy level increasing in quantum well.

A simple method was employed for the synthesis of green luminescent carbon quantumdots (CQDs) from styrene soot. The CQDs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, and Raman spectroscopy. The prepared carbon quantumdots did not show cellular toxicity and could successfully be used for labeling cells. We also evaluated the effects of carbon quantumdots on the process of angiogenesis. Results of a chorioallantoic membrane (CAM) assay revealed the significant decrease in the density of branched vessels after their treatment with CQDs. Further application of CQDs significantly downregulated the expression levels of pro-angiogenic growth factors like VEGF and FGF. Expression of VEGFR2 and levels of hemoglobin were also significantly lower in CAMs treated with CQDs, indicating that the CQDs inhibit angiogenesis. Data presented here also show that CQDs can selectively target cancer cells and therefore hold potential in the field of cancer therapy.

This work is focused on the optical dynamics of mesoscopic semiconductor heterostructures, using as prototypes zero-dimensional quantumdots and quantum cascade lasers which consist of quasitwo- dimensional quantum wells. Within a density matrix theory, a microscopic many-particle theory is applied to study scattering effects in these structures: the coupling to external as well as local fields, electron-phonon coupling, coupling to impurities, and Coulomb coupling. For both systems, the investigated effects are compared to experimentally observed results obtained during the past years. In quantumdots, the three-dimensional spatial confinement leads to the necessity to consider a quantum kinetic description of the dynamics, resulting in non-Markovian electron-phonon effects. This can be seen in the spectral phonon sidebands due to interaction with acoustic phonons as well as a damping of nonlinear Rabi oscillations which shows a nonmonotonous intensity and pulse duration dependence. An analysis of the inclusion of the self-interaction of the quantumdot shows that no dynamical local field terms appear for the simple two-level model. Considering local fields which have their origin in many quantumdots, consequences for a two-level quantumdot such as a zero-phonon line broadening and an increasing signal in photon echo experiments are found. For the use of quantumdots in an optical spin control scheme, it is found that the dephasing due to the electron-phonon interaction can be dominant in certain regimes. Furthermore, soliton and breather solutions are studied analytically in nonlinear quantumdot ensembles. Generalizing to quasi-two-dimensional structures, the intersubband dynamics of quantum cascade laser structures is investigated. A dynamical theory is considered in which the temporal evolution of the subband populations and the current density as well as the influence of scattering effects is studied. In the nonlinear regime, the scattering dependence and

This thesis explores the understanding of the chemistry and physics of colloidal quantumdots for practical solar energy photoconversion. Solar cell devices that make use of PbS quantumdots generally rely on constant and unchanged optical properties such that band gap energies remain tuned within the device. The design and development of unique experiments to ascertain mechanisms of optical band gap shifts occurring in PbS quantumdot thin-films exposed to air are discussed. The systematic study of the absorption properties of PbS quantumdot films exposed to air, heat, and UV illumination as a function of quantumdot size has been described. A method to improve the air-stability of films with atomic layer deposition of alumina is demonstrated. Encapsulation of quantumdot films using a protective layer of alumina results in quantumdot solids that maintain tuned absorption for 1000 hours. This thesis focuses on the use of atomic force microscopy and electrical variants thereof to study the physical and electrical characteristics of quantumdot arrays. These types of studies have broad implications in understanding charge transport mechanisms and solar cell device operation, with a particular emphasis on quantumdot transistors and solar cells. Imaging the channel potential of a PbSe quantumdot thin-film in a transistor showed a uniform distribution of charge coinciding with the transistor current voltage characteristics. In a second study, solar cell device operation of ZnO/PbS heterojunction solar cells was investigated by scanning active cross-sections with Kelvin probe microscopy as a function of applied bias, illumination and device architecture. This technique directly provides operating potential and electric field profiles to characterize drift and diffusion currents occurring in the device. SKPM established a field-free region occurring in the quantumdot layer, indicative of diffusion-limited transport. These results provide the path to optimization of

We report the fabrication of ITO/n-ZnO NW/p-Si sandwiched structure and its photoelectric and piezoelectric conversion properties. This hybrid cell was designed to harvest simultaneously both solar and mechanical energies. ZnO nanowires used in the work were grown on p-type Si substrates employing seed mediated low-temperature aqueous solution method. The synthesized ZnO nanowires were characterized by XRD, SEM and EDX characterization for their structural and morphological evaluation. The as-grown ZnO nanowires showed good crystallinity with c-axis preferable orientation. Free ZnO nanowires and CdSe quantumdots were also incorporated with the vertically grown nanowires and their response in harvesting optical and mechanical energies were investigated. The piezoelectric and photoelectric coupled effects of a ZnO nanowire device in the simultaneous conversion of both optical and mechanical energies have been studied for the first time with the goal of designing piezoelectric and photoelectric hybrid nanogenerator. This presented ITO/n-ZnO NW/p-Si heterojunction architecture is envisaged as a potentially valuable candidate for the next generation energy harvesting devices. Graphene-coated ITO was also used and its response was studied. - Highlights: ► Fabricated an unique sandwich type ITO/n-ZnO NW/p-Si nanogenerator. ► Simultaneous harvesting of both light and mechanical energy achieved. ► CdSe incorporation with ZnO nanowires increased the performance of the device. ► First report integrated both nanogenerator and solar cell in a same surface. ► Incorporating nanoantenna this structure can harvest light, heat and sound.

The two lowest single-particle hole states in the vertically coupled Ge/Si double layer quantumdots are investigated numerically by using the single-band heavy hole effective mass approximation and six-band Kronig-Penney model, re-spectively. The calculated results indicate that within the frame of several-band coupled model, the bonding-antibonding ground-state transition and a bonding-antibonding energy anti-crossover phenomenon are observed with interdot dis-tance increasing. These results have not been observed previously in those single-band model calculations. The analysis of the wavefunction component of bonding-antibonding hole state shows that the contribution ratios of light, heavy and spin-orbital-split-off hole states to the characteristic hole wavefunction vary with the increase of the vertical coupled distance, resulting in the ground state wavefunction changing from bonding states to antibonding ones finally.%分别采用单带重空穴近似和六带Kronig-Penney模型，对垂直耦合锗量子点在不同耦合距离下的空穴态特性进行了计算，并探讨了自旋-轨道的相互作用对空穴态对称性的影响。计算结果表明：多带耦合的框架下，随着量子点垂直间距的增大，空穴基态从成键态转变为反键态，而且价带基态能级和第一激发态能级发生反交叉现象，这与单带模型下得到的相应结果存在较大差异。通过分析六带模型计算得到的成、反键态波函数，轻、重空穴态和自旋-轨道分裂态对特征空穴态波函数的贡献比例随着量子点垂直间距的增大发生了转变，并最终导致量子点空穴基态波函数由成键态转变为反键态。

Realistic relaxed configurations of triaxially strained graphene quantumdots are obtained from unbiased atomistic mechanical simulations. The local electronic structure and quantum transport characteristics of y-junctions based on such dots are studied, revealing that the quasi-uniform pseudomagnetic field induced by strain restricts transport to Landau level- and edge state-assisted resonant tunneling. Valley degeneracy is broken in the presence of an external field, allowing the selective filtering of the valley and chirality of the states assisting in the resonant tunneling. Asymmetric strain conditions can be explored to select the exit channel of the y-junction.

We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantumdot devices. At temperatures below 500 mK, the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantumdot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.

This article reviews the application of high resolution scanning transmission electron microscopy (STEM) to semiconductor quantumdots (QDs) and quantum wires (QWRs). Different imaging and analytical techniques in STEM are introduced and key examples of their application to QDs and QWRs...

We demonstrate an instantaneous all-optical manipulation of optical absorption at the ground state of InGaAs/GaAs quantumdots (QDs) via a quantum-confined Stark effect (QCSE) induced by the electric field of incident THz pulses with peak electric fields reaching 200 kV/cm in the free space...

The polaron effect in a quantumdotquantum well (QDQW)system is investigated by using the perturbation method. Both the bound electron states outside and inside the shell well are taken into account . Numerical calculation on the CdS/HgS QDQW shows that the phonon correction to the electron ground state energy is quite significant and cannot be neglected.

We present an overview of experimental steps taken towards using the spin of a single electron trapped in a semiconductor quantumdot as a spin qubit [Loss and DiVincenzo, Phys. Rev. A 57, 120 (1998)]. Fabrication and characterization of a double quantumdot containing two coupled spins has been achieved, as well as initialization and single-shot read-out of the spin state. The relaxation time T {sub 1} of single-spin and two-spin states was found to be on the order of a millisecond, dominated by spin-orbit interactions. The time-averaged dephasing time T{sub 2}{sup *}, due to fluctuations in the ensemble of nuclear spins in the host semiconductor, was determined to be on the order of several tens of nanoseconds. Coherent manipulation of single-spin states can be performed using a microfabricated wire located close to the quantumdot, while two-spin interactions rely on controlling the tunnel barrier connecting the respective quantumdots [Petta et al., Science 309, 2180 (2005)]. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

photosynthetic systems. The next paper, by Olejnik et al, discussed metallic QDs which enhance photosynthetic function in light-harvesting biomolecular complexes. Such hybrid structures with gold QDs are shown to exhibit a strong increase in the fluorescence quantum yield. The next two papers, by Sikora et al and Kaminska et al deal with the ZnO nanoparticles passivated by MgO. In the first of these two papers, the authors describe the behavior of ZnO/MgO when introduced to human cancer cells. In the second, the authors describe the QDs with an extra outer layer of Fe2O3 which makes the nanoparticles superparamagnetic and also capable of generation of reactive oxygen species which could be applied to form localized centers of toxicity for cancer treatment. Finally, in the last paper by Yatsunenko et al, the authors discuss several semiconducting QDs like ZnO with various rare-earth dopands. They propose a microwave-driven hydrothermal technology to make them, characterize their luminescence and demonstrate their usefulness in the early recognition of cancer tissues. Quantumdots as probes in biology contents Quantumdots as probes in biologyMarek Cieplak Luminescent nanoparticles and their applications in the life sciencesVarun K A Sreenivasan, Andrei V Zvyagin and Ewa M Goldys Ferredoxin:NADP+ oxidoreductase in junction with CdSe/ZnS quantumdots: characteristics of an enzymatically active nanohybrid Krzysztof Szczepaniak, Remigiusz Worch and Joanna Grzyb Spectroscopic studies of plasmon coupling between photosynthetic complexes and metallic quantumdotsMaria Olejnik, Bartosz Krajnik, Dorota Kowalska, Guanhua Lin and Sebastian Mackowski Luminescence of colloidal ZnO nanoparticles synthesized in alcohols and biological application of ZnO passivated by MgOBożena Sikora, Krzysztof Fronc, Izabela Kamińska, Kamil Koper, Piotr Stępień and Danek Elbaum Novel ZnO/MgO/Fe2O3 composite optomagnetic nanoparticles I Kamińska, B Sikora, K Fronc, P Dziawa, K Sobczak, R Minikayev, W

We report photoluminescence emission from single self-assembled InAlGaAs quantumdots, which is broadened purely by dephasing processes. We observe linewidths as low as 6+/-3@meV at 10K, which agrees with the homogeneous linewidth derived from four-wave mixing experiments. By selecting dots...... that are not affected by local field fluctuations and using high-energy excitation, we avoid additional sources of linewidth broadening typically present in single-dot photoluminescence spectra. We observe a strong LO-phonon coupling in InAlGaAs and InGaAs dots, which becomes the dominating contribution...

The coupling between a quantumdot (QD) and a micropillar cavity is experimentally investigated by performing time-resolved, correlation, and two-photon interference measurements. The Jaynes-Cummings model including dissipative Lindblad terms and dephasing is analyzed, and all the parameters...... for the model are experimentally determined allowing for a complete comparison between experiment and theory....

The coupling between a quantumdot (QD) and a micropillar cavity is experimentally investigated by performing time-resolved, correlation, and two-photon interference measurements. The Jaynes-Cummings model including dissipative Lindblad terms and dephasing is analyzed, and all the parameters...

We study theoretically the dynamical response of a set of solid-state quantum emitters arbitrarily coupled to a single-mode microcavity system. Ramping the matter-field coupling strength in round trips, we quantify the hysteresis or irreversible quantum dynamics. The matter-field system is modeled as a finite-size Dicke model which has previously been used to describe equilibrium (including quantum phase transition) properties of systems such as quantumdots in a microcavity. Here we extend this model to address non-equilibrium situations. Analyzing the system’s quantum fidelity, we find that the near-adiabatic regime exhibits the richest phenomena, with a strong asymmetry in the internal collective dynamics depending on which phase is chosen as the starting point. We also explore signatures of the crossing of the critical points on the radiation subsystem by monitoring its Wigner function; then, the subsystem can exhibit the emergence of non-classicality and complexity.

We study theoretically the dynamical response of a set of solid-state quantum emitters arbitrarily coupled to a single-mode microcavity system. Ramping the matter-field coupling strength in round trips, we quantify the hysteresis or irreversible quantum dynamics. The matter-field system is modeled as a finite-size Dicke model which has previously been used to describe equilibrium (including quantum phase transition) properties of systems such as quantumdots in a microcavity. Here we extend this model to address non-equilibrium situations. Analyzing the system's quantum fidelity, we find that the near-adiabatic regime exhibits the richest phenomena, with a strong asymmetry in the internal collective dynamics depending on which phase is chosen as the starting point. We also explore signatures of the crossing of the critical points on the radiation subsystem by monitoring its Wigner function; then, the subsystem can exhibit the emergence of non-classicality and complexity.

Silicon nanocrystals represent an important class of non-toxic, heavy-metal free quantumdots, where the high natural abundance of silicon is an additional advantage. Successful development in mass-fabrication, starting from porous silicon to recent advances in chemical and plasma synthesis, opens up new possibilities for applications in optoelectronics, bio-imaging, photovoltaics, and sensitizing areas. In this review basic physical properties of silicon nanocrystals revealed by photoluminescence spectroscopy, lifetime, intensity trace and electrical measurements on individual nanoparticles are summarized. The fabrication methods developed for accessing single Si nanocrystals are also reviewed. It is concluded that silicon nanocrystals share many of the properties of direct bandgap nanocrystals exhibiting sharp emission lines at low temperatures, on/off blinking, spectral diffusion etc. An analysis of reported results is provided in comparison with theory and with direct bandgap material quantumdots. In addition, the role of passivation and inherent interface/matrix defects is discussed.

Transport properties of a gated nanostructure depend crucially on the coupling of its states to the states of electrodes. In the case of a single quantumdot the coupling, for a given quantum state, is constant or can be slightly modified by additional gating. In this paper we consider a concentric dot-ring nanostructure (DRN) and show that its transport properties can be drastically modified due to the unique geometry. We calculate the dc current through a DRN in the Coulomb blockade regime and show that it can efficiently work as a single-electron transistor (SET) or a current rectifier. In both cases the transport characteristics strongly depend on the details of the confinement potential. The calculations are carried out for low and high bias regime, the latter being especially interesting in the context of current rectification due to fast relaxation processes.

Full Text Available The Bose-Einstein condensation temperature Tc for a system of coupledquantumdots in a microcavity was estimated in function of the confining potential steepness, the external magnetic field strength, and the barrier layer width for indirect excitons. The effect of the magnetic field on Tc was found to be nonmonotonic over a certain range of the control parameters. The reason is the presence of two competing mechanisms accompanying the increase of the magnetic field: (a increase of the magnetoexciton effective mass and (b increase of the effective confining potential steepness for quantumdots.

Many processes of interest in quantumdots involve charge or energy transfer from one dot to another. Energy transfer in films of quantumdots as well as between linked quantumdots has been demonstrated by luminescence shift, and the ultrafast time-dependence of energy transfer processes has been resolved. Bandgap variation among dots (energy disorder) and dot separation are known to play an important role in how energy diffuses. Thus, it would be very useful if energy transfer could be visualized directly on a dot-by-dot basis among small clusters or within films of quantumdots. To that effect, we report single molecule optical absorption detected by scanning tunneling microscopy (SMA-STM) to image energy pooling from donor into acceptor dots on a dot-by-dot basis. We show that we can manipulate groups of quantumdots by pruning away the dominant acceptor dot, and switching the energy transfer path to a different acceptor dot. Our experimental data agrees well with a simple Monte Carlo lattice model of energy transfer, similar to models in the literature, in which excitation energy is transferred preferentially from dots with a larger bandgap to dots with a smaller bandgap.

Interacting fermions on a lattice can develop strong quantum correlations, which are the cause of the classical intractability of many exotic phases of matter. Current efforts are directed towards the control of artificial quantum systems that can be made to emulate the underlying Fermi-Hubbard models. Electrostatically confined conduction-band electrons define interacting quantum coherent spin and charge degrees of freedom that allow all-electrical initialization of low-entropy states and readily adhere to the Fermi-Hubbard Hamiltonian. Until now, however, the substantial electrostatic disorder of the solid state has meant that only a few attempts at emulating Fermi-Hubbard physics on solid-state platforms have been made. Here we show that for gate-defined quantumdots this disorder can be suppressed in a controlled manner. Using a semi-automated and scalable set of experimental tools, we homogeneously and independently set up the electron filling and nearest-neighbour tunnel coupling in a semiconductor quantumdot array so as to simulate a Fermi-Hubbard system. With this set-up, we realize a detailed characterization of the collective Coulomb blockade transition, which is the finite-size analogue of the interaction-driven Mott metal-to-insulator transition. As automation and device fabrication of semiconductor quantumdots continue to improve, the ideas presented here will enable the investigation of the physics of ever more complex many-body states using quantumdots.

Interacting fermions on a lattice can develop strong quantum correlations, which are the cause of the classical intractability of many exotic phases of matter. Current efforts are directed towards the control of artificial quantum systems that can be made to emulate the underlying Fermi-Hubbard models. Electrostatically confined conduction-band electrons define interacting quantum coherent spin and charge degrees of freedom that allow all-electrical initialization of low-entropy states and readily adhere to the Fermi-Hubbard Hamiltonian. Until now, however, the substantial electrostatic disorder of the solid state has meant that only a few attempts at emulating Fermi-Hubbard physics on solid-state platforms have been made. Here we show that for gate-defined quantumdots this disorder can be suppressed in a controlled manner. Using a semi-automated and scalable set of experimental tools, we homogeneously and independently set up the electron filling and nearest-neighbour tunnel coupling in a semiconductor quantumdot array so as to simulate a Fermi-Hubbard system. With this set-up, we realize a detailed characterization of the collective Coulomb blockade transition, which is the finite-size analogue of the interaction-driven Mott metal-to-insulator transition. As automation and device fabrication of semiconductor quantumdots continue to improve, the ideas presented here will enable the investigation of the physics of ever more complex many-body states using quantumdots.

Semiconductor quantumdots are considered a promising material system for future optical devices and quantum computers. We have studied the low-temperature photoluminescence properties of single InGaAs quantumdots embedded in GaAs. The high spatial resolution required for resolving single dots...... to resolve luminescence lines from individual quantumdots, revealing an atomic-like spectrum of sharp transition lines. A parameter of fundamental importance is the intrinsic linewidth of these transitions. Using high-resolution spectroscopy we have determined the linewidth and investigated its dependence...... on temperature, which gives information about how the exciton confined to the quantumdot interacts with the surrounding lattice....

Motivated by recent interest in implementing circuit quantum electrodynamics with semiconducting quantumdots, we consider a double quantumdot (DQD) capacitively coupled to a superconducting resonator that is driven by the microwave field of a superconducting transmission line. We analyze the DQD current response using input-output theory and show that the resonator-coupled DQD is a sensitive microwave single photon detector. Using currently available experimental parameters of DQD-resonator coupling and dissipation, including the effects of 1 /f charge noise and phonon noise, we determine the parameter regime for which incident photons are completely absorbed and near-unit ≳98 % efficiency can be achieved. We show that this regime can be reached by using very high quality resonators with quality factor Q ≃105 .

The main goal of the project has been to realize an efficient source of coherent single photons by coupling a self-assembled quantumdot to a photonic crystal waveguide. Such a source would have a wide range of applications in the field of quantum information processing. By studying the coupling...... on this result, we improved the design of the photonic crystal waveguide, and we characterized in detail the efficiency of the device and the coherence of the emitted single photons. We investigate the decoherence mechanisms affecting the quantumdots by performing resonance fluorescence experiments on emitters...... of a single charge. A very high degree of coherence can be achieved by embedding quantumdots in electrically gated samples. We show that a single quantumdot behaves like a nearly-ideal two-level system in a sample with electrical gates, and single photons emitted up to 1 μs apart show indistinguishability...

Three different size distributions of Ge quantumdots (≳200, 110, and 60 Å) have been synthesized via the ultrasonic mediated reduction of mixtures of chlorogermanes and organochlorogermanes (or organochlorosilanes) by a colloidal sodium/potassium alloy in heptane, followed by annealing in a sealed pressure vessel at 270 °C. The quantumdots are characterized by transmission electron microscopy, x-ray powder diffraction, x-ray photoemission, infrared spectroscopy, and Raman spectroscopy. Colloidal suspensions of these quantumdots were prepared and their extinction spectra are measured with ultraviolet/visible (UV/Vis) and near infrared (IR) spectroscopy, in the regime from 0.6 to 5 eV. The optical spectra are correlated with a Mie theory extinction calculation utilizing bulk optical constants. This leads to an assignment of three optical features to the E(1), E(0'), and E(2) direct band gap transitions. The E(0') transitions exhibit a strong size dependence. The near IR spectra of the largest dots is dominated by E(0) direct gap absorptions. For the smallest dots the near IR spectrum is dominated by the Γ25→L indirect transitions.

Understanding the dynamics of open quantum systems is important and challenging in basic physics and applications for quantum devices and quantum computing. Semiconductor quantumdots offer a good platform to explore the physics of open quantum systems because we can tune parameters including the coupling to the environment or leads. Here, we apply the fast single-shot measurement techniques from spin qubit experiments to explore the spin and charge dynamics due to tunnel coupling to a lead in a quantumdot-lead hybrid system. We experimentally observe both spin and charge time evolution via first- and second-order tunneling processes, and reveal the dynamics of the spin-flip through the intermediate state. These results enable and stimulate the exploration of spin dynamics in dot-lead hybrid systems, and may offer useful resources for spin manipulation and simulation of open quantum systems.

Recent experimental developments in the field of semiconductor quantumdot (QD) spectroscopy are discussed. Firstly, we report about single QD exciton two-level systems and their coherent properties in terms of single-qubit manipulations. In the second part, we report on coherent quantumcoupling in a prototype 'two-qubit' system consisting of a vertically stacked pair of QDs. The interaction can be tuned in such QD molecule devices using an applied voltage as external parameter.

Recent experimental developments in the field of semiconductor quantumdot (QD) spectroscopy are discussed. Firstly, we report about single QD exciton two-level systems and their coherent properties in terms of single-qubit manipulations. In the second part, we report on coherent quantumcoupling in a prototype 'two-qubit' system consisting of a vertically stacked pair of QDs. The interaction can be tuned in such QD molecule devices using an applied voltage as external parameter.

Two quantum control spectroscopic techniques were applied to study InAs quantumdot (QD) devices, which contain different strain-reducing layers. By adaptively control light matter interaction, a delayed resonant response from the InAs QDs was found to be encoded into the optimal phase profile of ultrafast optical pulse used. We verified the delayed resonant response to originate from excitons coupled to acoustic phonons of InAs QDs with two-dimensional coherent spectroscopy. Our study yields valuable dynamical information that can deepen our understanding of the coherent coupling process of exciton in the quantum-confined systems.

CdSe QuantumDots (QDs) of size 0.85 nm were synthesized using chemical route. ZnO based QuantumDot Sensitized Solar Cell (QDSSC) was fabricated using CdSe QDs as sensitizer. The Pre-synthesized QDs were found to be successfully adsorbed on front ZnO electrode and had potential to replace organic dyes in Dye Sensitized Solar Cells (DSSCs). The efficiency of QDSSC was obtained to be 2.06 % at AM 1.5.

It is known experimentally that stable charged-exciton complexes can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to be solved, even numerically. Here we introduce the correlated hyperspherical harmonics as basis functions to solve the hyperangular equation for negatively and positively charged excitons (trions) in a harmonic quantumdot. By using this method, we have calculated the energy spectra of the low-lying states of a charged exciton as a function of the radius of quantumdot. Based on symmetry analysis, the level crossover as the dot radius increases can be fully explained as the results of symmetry constraint.``

Optical bistability in quantumdots, recently proposed by Chemla and Miller, is studied in a two-resonance model. We show that for such classical electromagnetic resonances the applicability of a two-resonance model is far more restrictive than for those in atoms.

Embodiments of the present disclosure provide for methods of making quantumdots (QDs) (passivated or unpassivated) using a continuous flow process, systems for making QDs using a continuous flow process, and the like. In one or more embodiments, the QDs produced using embodiments of the present disclosure can be used in solar photovoltaic cells, bio-imaging, IR emitters, or LEDs.